Storage management device, storage system control device, storage medium storing storage management program, and storage system

ABSTRACT

A storage management device includes a management unit for managing a storage device assigned thereto and a temporary storage unit assigned thereto. The management unit comprises a backup unit for, when data is written into the storage device, storing the data in the previously assigned temporary storage unit before the writing of the data into the storage device is completed, and a take-over unit for, when data which is already stored in the temporary storage unit, but which is not yet written into the storage device exists when the storage device and the temporary storage unit are assigned, writing the not-yet-written data into the storage device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit of priority from Japanese Patent Application No. 2007-314706, filed on Dec. 5, 2007, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a storage management device, a storage system control device, a storage medium storing a storage management program, and a storage system.

SUMMARY OF THE INVENTION

In keeping with one aspect of the present invention, there is provided a storage management device for a storage system, that stores data in a plurality of storage devices interconnected via a network in a distributed manner. The storage management device includes a management unit for managing the storage device assigned thereto, and a temporary storage unit assigned thereto. The management unit has a backup unit for, when the data is written into the storage device, storing the data in the previously assigned temporary storage unit before the writing of the data into the storage device is completed. When data is already stored in the temporary storage unit, but is not yet written into the storage device when the storage device and the temporary storage unit are assigned, a take-over unit writes the not-yet-written data into the storage device.

Additional aspects and/or advantages will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention with reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the outline of an embodiment;

FIG. 2 illustrates the system configuration of the embodiment;

FIG. 3 is a block diagram showing the hardware configuration of a storage node;

FIG. 4 is a block diagram showing the functions of the storage node;

FIG. 5 illustrates the data structure of a connection state table;

FIG. 6 illustrates one example of assignment setting when a storage system is started up;

FIG. 7 illustrates one example of the assignment setting when the storage node has failed;

FIG. 8 illustrates one example of the assignment setting when a cache memory has failed;

FIG. 9A is a table for explaining update of the connection state table when the storage node has failed;

FIG. 9B is a table for explaining the update of the connection state table when the storage node has failed;

FIG. 10A is a table for explaining update of the connection state table when the cache memory has failed;

FIG. 10B is a table for explaining the update of the connection state table when the cache memory has failed;

FIG. 11 is a sequence chart showing the procedures of an assignment process at startup of the storage system;

FIG. 12 is a sequence chart showing the procedures of a write process in an ordinary mode;

FIG. 13 is a sequence chart showing the procedures of the write process when the cache memory has no vacancy;

FIG. 14 is a sequence chart showing the procedures of the assignment process when the storage node has failed;

FIG. 15 is a sequence chart showing the procedures of the assignment process when the cache memory has failed;

FIG. 16 is a sequence chart showing the procedures of a process of performing change from the ordinary mode to a synchronous mode;

FIG. 17 is a sequence chart showing the procedures of a synchronous write process executed by the storage node;

FIG. 18 is a sequence chart showing the procedures of a process of performing change from the synchronous mode to the ordinary mode;

FIG. 19 is a block diagram showing an exemplary configuration of the storage system according to the embodiment;

FIG. 20 illustrates the assignment setting in the exemplary configuration of the storage system according to the embodiment in a usual state;

FIG. 21 illustrates the assignment setting in the exemplary configuration of the storage system according to the embodiment when the storage node has failed;

FIG. 22A illustrates a connection state table in the exemplary configuration of the storage system according to the embodiment when the storage system is in the usual state;

FIG. 22B illustrates a connection state table in the exemplary configuration of the storage system according to the embodiment when the storage node has failed; and

FIG. 23 is a block diagram showing an exemplary configuration of a storage system according to a modification of the embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With the widespread use of data processing employing computers, storage techniques for accumulating and utilizing data have become more important. Hitherto, RAID (Redundant Arrays of Independent Disks) has been generally employed as one storage technique for realizing a higher speed of data access and higher reliability in data storage. According to RAID, data is divided and copied, as required, to be distributed into a plurality of disk devices. Thus, a higher speed is realized with distribution of loads among the plurality of disk devices, and higher reliability is realized with redundancy of data.

Recently, a distributed storage system utilizing the concept of RAID has been constructed to achieve an even higher speed and even higher reliability. The distributed storage system includes a plurality of storage nodes and a network interconnecting the storage nodes. Each storage node manages an associated (corresponding) disk device which is internally or externally disposed, and it has a network communication function. In the distributed storage system, data is distributed and stored in the plurality of storage nodes, whereby an even higher speed and even higher reliability are realized in the entire system.

When data is redundantly stored in a distributed storage system, i.e., when data having the same contents is arranged in a plurality of storage nodes, a failure can occur in any of individual storage nodes in practice.

Regarding a system recovery in the event of such a failure, a distributed storage system in which a logical disk is constituted by a plurality of dual segments and is distributed to a plurality of storage nodes is known. In the known distributed storage system, if one storage node has failed, the data in one of the dual segments which is not destroyed is copied to another storage node so that the dual state is restored (see, e.g., Japanese Unexamined Patent Application Publication No. 2005-4681). Thus, according to the known distributed storage system, because a plurality of segments are distributed to a plurality of storage nodes, a recovery to the dual state after the occurrence of a failure can be performed in parallel in units of plural segments and the recovery can be promptly achieved.

Also, there is known a storage system in which a plurality of storage nodes constitute an interconnected network, and storage areas distributed to the plurality of storage nodes are joined by some storage device into one storage area for access by a host computer. A disk cache is constituted in dual redundancy to increase durability against medium failures (see, e.g., Japanese Unexamined Patent Application Publication No. 2005-165702). According to such a storage system, because the storage nodes are interconnected via the network, each storage node can access another storage node.

Further, in a storage system including a plurality of storage nodes and a control unit for selecting the storage node in use with the intent to even out the frequency of use of each cache memory, it is proposed to constitute a disk cache in the control unit in dual redundancy to prevent loss of data not yet reflected on a disk device (see, e.g., Japanese Unexamined Patent Application Publication No. 2005-275525).

With a further known technique, a disk control device includes plural systems of cache memories which are independent of one another. Backup data of an updated disk, not-yet-updated data, and backup data of the not-yet-updated data are loaded into the respective systems of cache memories such that the not-yet-updated data in the cache memory can be restored (see, e.g., Japanese Unexamined Patent Application Publication No. 10-198602).

In the storage node using the multi-node storage such as described in Japanese Unexamined Patent Application Publication No. 2005-4681, however, a storage device (disk) and a control unit for controlling the storage device are integrated with each other. Also, the known storage node is generally constructed such that a disk array device based on, e.g., a RAID system, is used as a disk, a computer, e.g., a PC server, is used as a control server, and the disk array device and the PC server are fixedly interconnected by, e.g., SCSI (Small Computer System Interface) connection.

In the above-described configuration, if the control server has failed, a client cannot access data in the storage device in spite of the storage device being normal. Accordingly, data needs to be stored in dual redundancy by using the method disclosed in Japanese Unexamined Patent Application Publication No. 2005-4681, for example, so that the system can be recovered in the event of a failure of the control unit.

However, storing the data in dual redundancy increases the cost of the multi-node storage. Another problem is that, when a recovery is performed, the system performance is reduced during the recovery.

Recently, with development of a highly reliable technique represented by RAID6, for example, the disk array technology of RAID or the like has become sufficiently reliable even in the single form without needing dual redundancy. Instead of fixedly connecting the storage device using a disk array to the PC server by the SCSI connection, for example, the storage device can be connected to the PC server via a storage area network using, e.g., a fiber channel or an iSCSI (Internet Small Computer System Interface). With such a configuration, even if the PC server has failed, access to data in the storage device can be continued by connecting the storage device to another PC server which serves as a control unit of another storage node. Thus, reliability can be ensured without needing the dual redundancy and the recovery.

However, data temporarily stored in a memory of one PC server and not yet written into the storage device cannot be taken over to another PC server even with succession of the storage device. All data can be taken over by synchronously writing all the data into the storage device RAID, but the synchronous writing greatly reduces a response time because the completion of the writing is not notified to a client having requested the writing of the data until the writing of the data into the storage device is completed.

In the storage system described in Japanese Unexamined Patent Application Publication No. 2005-165702, if a control unit constituting one storage node has failed, another storage node cannot access a disk in the one storage node. Therefore, even when only the control unit for controlling the disk of the storage node has failed, access to data stored in the disk of the relevant storage node can no longer be performed. Further, while Japanese Unexamined Patent Application Publication No. 2005-165702 describes the disk cache in dual redundancy, only the disk cache in the storage node is constituted in dual redundancy. This causes another problem because if the storage node has failed, all data in the disk cache of the storage node is lost.

In the storage system described in Japanese Unexamined Patent Application Publication No. 2005-275525, an external disk can be connected to the storage control unit. However, if the control unit has failed, access to the external disk is disabled because no countermeasures are taken into account against such an event. Further, as in Japanese Unexamined Patent Application Publication No. 2005-165702, while Japanese Unexamined Patent Application Publication No. 2005-275525 describes the disk cache in dual redundancy in the storage system, only the disk cache in the control unit is constituted in dual redundancy. If the control unit has failed, all data in the disk cache is lost.

In the technique described in Japanese Unexamined Patent Application Publication No. 10-198602, similarly to the above-described related art, the cache memory is just constituted in dual redundancy in a single device. This also causes the problem that, if the single device itself has failed, all data in the cache memory is lost.

An embodiment of the present invention will be described in detail below with reference to the drawings. Firstly the outline of the present invention will be described and thereafter the specific content of the embodiment will be described.

FIG. 1 illustrates the outline of the embodiment. A storage system shown in FIG. 1 distributes and stores data in a plurality of storage devices interconnected via a network. The storage system is constituted by a storage management device 1, a storage device 2, a temporary storage unit (means) 3, and a terminal device 4.

The storage management device 1 manages the storage device 2 and the temporary storage unit 3. The storage management device 1 includes a management unit 10. The management unit 10 includes a backup unit 10 a and a take-over unit 10 b for managing the storage device 2 previously assigned thereto and the temporary storage unit 3 previously assigned thereto.

When data is written into the storage device 2 in response to a request from the terminal device 4, for example, the backup unit 10 a stores the data, which is to be written into the storage device 2, in the previously-assigned temporary storage unit 3 as well before the data transmitted from the terminal device 4 is completely written into the storage device 2. The temporary storage unit 3 is to temporarily store data for backup, and it can be a storage device such as a cache memory.

Assignment of the storage device 2 and the temporary storage unit 3 to the storage management device 1 can be performed in a manual manner in which an administrator appropriately assigns them, or in an automatic manner in which the storage management device 1 itself or another device assigns them according to a particular policy based on, e.g., load balance.

If there is data which is stored in the temporary storage unit 3, but which is not yet written into the storage device 2 when the storage device 2 and the temporary storage unit 3 are newly assigned to the storage management device 1, the take-over unit 10 b writes that data into the storage device 2. The take-over unit 10 b and the storage management device 1 can realize the above-mentioned process by various methods. In one example of the various methods, after the storage management device 1 has completed writing of data received from the terminal device 4 into the storage device 2, the data stored in the temporary storage unit 3 by the backup unit 10 a is erased. With such erasing, data in the progress of writing into the storage device 2, which is set in a corresponding relation to the temporary storage unit 3 by the assignment, is stored in the temporary storage unit 3, whereas the data already reflected is erased. As a result, only the not-yet-reflected data is stored in the temporary storage unit 3. Thereafter, when the storage device 2 and the temporary storage unit 3 are newly assigned, the take-over unit 10 b writes the data stored in the temporary storage unit 3 into the storage device 2. In another example, a table is prepared to load data stored in the storage device 2 and the temporary storage unit 3, which correspond to each other by assignment. At the time of new assignment, the table is checked and data stored (updated) in only the temporary storage unit 3 is reflected on the storage device 2. Other suitable methods are also usable.

Thus, with the storage management device 1 described above, when data is written into the storage device 2 in response to a request from the terminal device 4, for example, the backup unit 10 a stores the data, which is to be written into the storage device 2, in the previously-assigned temporary storage unit 3 as well, before the data transmitted from the terminal device 4 is completely written into the storage device 2. Also, if there is data which is stored in the temporary storage unit 3, but which is not yet written into the storage device 2 when the storage device 2 and the temporary storage unit 3 are newly assigned to the storage management device 1, the take-over unit 10 b writes that data into the storage device 2.

Accordingly, the data not yet written into the storage device 2 in the event of a failure of some storage management device can be passed over to another normal storage management device 1, which is newly assigned, with backup executed by the temporary storage unit 3. The other data already written into the storage device 2 by the failed storage management device can also be passed over to the other normal storage management device 1 by new assignment of the storage device 2 and the temporary storage unit 3.

The present embodiment will be described in detail below with reference to the drawings.

FIG. 2 illustrates the system configuration of the embodiment. A storage system shown in FIG. 2 distributes and stores data in a plurality of storage devices interconnected via a network.

The storage system according to this embodiment includes a plurality (e.g., four) of storage nodes 100 a, 100 b, 100 c and 100 d, a plurality (e.g., four) of storage devices 200 a, 200 b, 200 c and 200 d, at least one (e.g., two) control nodes 500 a and 500 b, and at least one (e.g., one) management node 300, which are interconnected via a network 30. At least one terminal device, e.g., three terminal devices 400 a, 400 b and 400 c, are connected to the network 30 via a network 40.

The storage devices 200 a to 200 d are connected respectively to the storage nodes 100 a to 100 d via the network 30. The storage nodes 100 a to 100 d are computers for managing data loaded into the respectively assigned storage devices 200 a to 200 d and providing the data under the management to the terminal devices 400 a to 400 c via the networks 30 and 40. Thus, each storage node functions as a storage management device.

Also, the storage nodes 100 a to 100 d provide, to the terminal devices 400 a to 400 c, information processing service utilizing data loaded in the storage devices 200 a to 200 d which are managed respectively by the storage nodes 100 a to 100 d. More specifically, the storage nodes 100 a to 100 d execute predetermined programs in response to requests from the terminal devices 400 a to 400 c via the networks 30 and 40 to thereby perform reading/writing of data and transmission/reception of data in response to requests from the terminal devices 400 a to 400 c.

Further, though described in detail later, the storage nodes 100 a to 100 d have respective cache memories (not shown). Each of the cache memories functions as a temporary storage unit which can be assigned to any of other ones of the storage nodes 100 a to 100 d than the storage node that has the relevant cache memory, and which stores data transmitted from the storage node corresponding to the assigned cache memory and written into any of the storage devices 200 a to 200 d which is assigned to the storage node corresponding to the assigned cache memory.

In addition, upon receiving data read requests from the terminal devices 400 a to 400 c via the networks 30 and 40, the storage nodes 100 a to 100 d read the requested data from respective assigned ones of the storage devices 200 a to 200 d and then make a response to the terminal devices having transmitted the read requests. Further, upon receiving data write requests from the terminal devices 400 a to 400 c via the networks 30 and 40, the storage nodes 100 a to 100 d write the received data into the respective storage devices 200 a to 200 d and reply to the terminal devices having transmitted the data. Thereafter, the storage nodes 100 a to 100 d access the storage devices 200 a to 200 d and the cache memories which are connected to the respective storage nodes via the network 30.

The storage nodes 100 a to 100 d may provide data redundancy through mirroring such that at least two storage nodes manage the storage devices in different systems, which store the mirrored data having the same contents.

The storage devices 200 a to 200 d are each a disk array device which is constituted by a plurality of built-in hard disk drives (HDDs) using RAID. HDDs 201 a to 204 a are incorporated in the storage device 200 a. HDDs 201 b to 204 b are incorporated in the storage device 200 b. HDDs 201 c to 204 c are incorporated in the storage device 200 c. HDDs 201 d to 204 d are incorporated in the storage device 200 d. In this embodiment, the storage devices 200 a to 200 d provide disk management service of RAID5. Only the storage node assigned by the control nodes 500 a and 500 b can exclusively access corresponding ones of the storage devices 200 a to 200 d via the networks 30 and 40.

The storage devices 200 a to 200 d are not limited to RAID5 and they may be constituted by using another type of RAID. As an alternative, a disk array of the storage device may be constituted by using another suitable technique other than RAID. Further, the storage devices 200 a to 200 d may be each constituted by a single hard disk without using a disk array, or by another suitable storage device.

The control nodes 500 a and 500 b are computers for managing the storage nodes 100 a to 100 d and the storage devices 200 a to 200 d. More specifically, the control nodes 500 a and 500 b acquire information regarding data management from the storage nodes 100 a to 100 d and update the information as required.

To increase reliability, the control nodes 500 a and 500 b are constituted in dual redundancy such that one (e.g., 500 a) of the control nodes 500 a and 500 b is a currently operating system and is operated in the usual state, and that the other (e.g., the control node 500 b) is a standby system and is held standby in the usual state. For example, when the control node 500 a serving as the currently operating system has failed, the control node 500 b maintained as the standby system is operated instead of the control node 500 a.

When the storage system is started up, the control nodes 500 a and 500 b assign the storage devices 200 a to 200 d and the cache memories to the storage nodes 100 a to 100 d.

Further, the control nodes 500 a and 500 b can detect a failure of the storage nodes 100 a to 100 d and a failure of the cache memories. Upon detecting a failure of the storage nodes 100 a to 100 d, the control nodes 500 a and 500 b newly assign the storage device and the cache memory, which have been so far assigned to the failed storage node, to another storage node.

In addition, each of the control nodes 500 a and 500 b holds, in the form of, e.g., a connection state table, the setting of assignment of the storage devices 200 a to 200 d and the cache memories to the storage nodes 100 a to 100 d. When the assignment is performed at the startup, or when the assignment is updated with a new assignment upon, e.g., detection of a failure, the control nodes 500 a and 500 b update the connection state table and record the latest assignment setting.

The management node 300 is a computer operated by an administrator of the storage system. The administrator of the storage system can access the storage nodes 100 a to 100 d and the control nodes 500 a and 500 b, and further can perform various settings required for the operation by operating the management node 300.

The terminal devices 400 a to 400 c are computers operated by users of the storage system. The users of the storage system can access the storage nodes 100 a to 100 d and can perform reading/writing of data stored in the storage devices 200 a to 200 d by operating the terminal devices 400 a to 400 c.

The respective hardware configurations of the storage nodes 100 a to 100 d, the control nodes 500 a and 500 b, the terminal devices 400 a to 400 c, and the management node 300 will be described below.

FIG. 3 is a block diagram showing the hardware configuration of the storage node. The storage node 100 a is entirely controlled by a CPU (Central Processing Unit) 101 a. A RAM (Random Access Memory) 102 a, an HDD 103 a, a graphic processing device 104 a, an input interface 105 a, and a communication interface 106 a are connected to the CPU 101 a via a bus 107 a.

The RAM 102 a temporarily stores at least part of a program of an OS (Operating System) causing the CPU 101 a to run and execute application programs. The RAM 102 a also stores various data necessary for processing executed by the CPU 101 a. A portion of the RAM 102 a in the storage node 100 a is a cache memory 1021 a which can temporarily store data written into the storage devices 200 a to 200 d. The cache memory 1021 a functions as a temporary storage unit (means). While the cache memory 1021 a is arranged as a portion of the RAM 102 a in this embodiment, the arrangement of the cache memory 1021 a is not limited to that described in this embodiment. The cache memory 1021 a may be arranged separately from the RAM 102 a.

The programs of the OS and the applications are stored in the HDD 103 a. A monitor 11 a is connected to the graphic processing device 104 a. The graphic processing device 104 a displays an image on a screen of the monitor 11 a in accordance with an instruction from the CPU 101 a. A keyboard 12 a and a mouse 13 a are connected to the input interface 105 a. The input interface 105 a transmits signals sent from the keyboard 12 a and the mouse 13 a to the CPU 101 a via the bus 107 a.

The communication interface 106 a is connected to the network 30. The communication interface 106 a transmits and receives data to and from other computers, such as the control nodes 500 a and 500 b, via the network 30. Also, generally, one of the storage devices 200 a to 200 d interconnected via the network 30, which is assigned to the storage node 100 a, is connected to the storage node 100 a via the communication interface 106 a. The communication interface 106 a communicates with RAID controllers (not shown) built in the storage devices 200 a to 200 d to perform inputting/outputting of data to and from the storage devices 200 a to 200 d. The RAID controller in the storage device 200 a has the functions of RAID0 to RAID5.

The storage nodes 100 b to 100 d, the control nodes 500 a and 500 b, and the terminal devices 400 a to 400 c can also be each represented by a similar hardware configuration to that of the storage node 100 a. However, communication interfaces of the terminal devices 400 a to 400 c are connected to the network 40.

The module configuration of the storage nodes 100 a to 100 d will be described below.

FIG. 4 is a block diagram showing the functions of the storage node. While FIG. 4 shows the module configuration of the storage node 100 a, the storage nodes 100 b to 100 d can also be realized with a similar module configuration to that of the storage node 100 a.

The storage system of this embodiment is to distribute and store data in a plurality of storage devices which are interconnected via a network. In the storage system of this embodiment, each of the other storage nodes 100 b to 100 d also has the same functions as those of the storage node 100 a. Further, in the storage system of this embodiment, the storage nodes 100 a to 100 d and the control nodes 500 a and 500 b operate in a cooperated manner as described below. The following description is made of the storage nodes 100 a and 100 b, the storage device 200 a, the terminal device 400 a, and the control node 500 a, which are extracted as typical examples, as shown in FIG. 4, from the components of the storage system.

In FIG. 4, for the convenience of explanation, the network 30 and the network 40 are shown as one network. Also, the components of the storage system are interconnected, as required, via the network 30 and the network 40.

The storage node 100 a manages the storage device 200 a and a cache memory 1021 b described later. The storage node 100 a has a management unit 110 a. The management unit 110 a includes a backup unit 111 a and a take-over unit 112 a for managing the storage device 200 a and the cache memory 1021 b which are assigned thereto. Further, the storage node 100 a has a RAM 102 a. The storage node 100 a stores, in the RAM 102 a, data to be written into the storage device 200 a.

When data is written into the storage device 200 a in response to a request from the terminal device 400 a, for example, the backup unit 111 a stores not-yet-written data, of which writing to the storage device 200 a is not yet completed, in the previously assigned cache memory 1021 b as well before the data transmitted from the terminal device 400 a is completely written into the storage device 200 a. Thus, in the event of a failure of the storage node 100 a, because the data to be written into the storage device 200 a is stored in the cache memory 1021 b which is disposed externally of the storage node 100 a, the data can be written into the storage device 200 a by using the data stored in the cache memory 1021 b. As a result, loss of data caused by the failure of the storage node 100 a can be prevented.

The cache memory 1021 b is a storage unit for temporarily storing data for backup, and it can be a storage device such as an ordinary cache memory. After the writing of the data read out from the cache memory 1021 b into the storage device 200 a has been completed, the backup unit 111 a erases that data from the cache memory 1021 b. Accordingly, the data having been completely written into the storage device 200 a is erased from the cache memory 1021 b, while data of which writing into the storage device 200 a is not yet completed continues to remain in the cache memory 1021 b. As a result, when the storage node 100 a has failed, the presence or absence of the data of which writing into the storage device 200 a is not yet completed can be easily determined and specified.

Further, the backup unit 111 a stores the data, which is to be written into the storage device 200 a, in the RAM 102 a of the storage node 100 a as well before the completion of writing of the data. Accordingly, even with a failure generated in the storage node 100 b including the cache memory 1021 b, since the data to be written into the storage device 200 a is stored in the RAM 102 a, writing of the data into the storage device 200 a can be performed by using the data stored in the RAM 102 a. As a result, loss of data caused by the failure of the cache memory 1021 b can be prevented.

Still further, after the writing of the data into the storage device 200 a has been completed, the backup unit 111 a erases the data that has been written into the RAM 102 a. Accordingly, the data that has been completely written into the storage device 200 a is erased, while data of which writing into the storage device 200 a is not yet completed continues to remain in the RAM 102 a. As a result, when the cache memory 1021 b has failed, the presence or absence of the data of which writing into the storage device 200 a is not yet completed can be easily determined and specified.

In addition, after the writing of data into the cache memory 1021 b has been completed, but before the writing of the same data into the storage device 200 a is completed, the backup unit 111 a transmits a write completion response for notifying the completion of the writing of that data to the terminal device 400 a via the networks 30 and 40.

If there is data which is stored in the cache memory 1021 b, but which is not yet written into the storage device 200 a when the storage device 200 a and the cache memory 1021 b are newly assigned to the storage node 100 a by the control node 500 a, the take-over unit 112 a reads that data from the cache memory 1021 b and writes it into the storage device 200 a.

Also, if there is data which is stored in the RAM 102 a, but which is not yet written into the storage device 200 a when the cache memory 1021 b is newly assigned by the control node 500 a, the take-over unit 112 a reads that data from the RAM 102 a and writes it into the storage device 200 a. At that time, if the data having been written into the RAM 102 a by the backup unit 111 a remains, the take-over unit 112 a determines that not-yet-written data is present, and then writes the not-yet-written data into the storage device 200 a.

While the storage node 100 b is constructed similarly to the storage node 100 a and has the same function as the storage node 100 a, only those of the functions of the storage node 100 b which are necessary for explaining the operation of the storage node 100 a will be discussed in the following description. As in the storage node 100 a described above with reference to FIG. 3, the storage node 100 b has a RAM 102 b which is a volatile memory. A portion of the RAM 102 b is constituted as the cache memory 1021 b for temporarily storing the data to be written into the storage device 200 a, etc.

The storage device 200 a is a disk array device which is constituted, as described above, by the built-in HDDs 201 a to 204 a, and which provides disk management service of RAID5. Only the storage node 100 a assigned by the control node 500 a can exclusively access the storage device 200 a via the networks 30 and 40.

The control node 500 a is a computer for managing the storage nodes 100 a and 100 b and the storage device 200 a. The control node 500 a controls the storage system by managing the storage node 100 a that manages the storage device 200 a. In this embodiment, as described above, the control node is constituted in dual redundancy as the control nodes 500 a and 500 b. Herein, the control node 500 a is assumed to be operated as a currently operating system, and the control node 500 b held in the standby state as a standby system is omitted from the following description.

The control node 500 a includes a failure detection unit 501 a for detecting a failure of the storage node 100 a, and an assignment unit 502 a for assigning the storage device 200 a and the cache memory 1021 b to the storage node 100 a.

It is herein assumed that, at the startup of the storage system, the control node 500 a exclusively assigns the storage node 100 a to the storage device 200 a and the cache memory 1021 b. In other words, the storage device 200 a and the cache memory 1021 b are assigned such that the storage nodes other than the storage node 100 a cannot access the storage device 200 a and the cache memory 1021 b.

The failure detection unit 501 a can detect a failure of each of the storage nodes 100 a to 100 d constituting the storage system. More specifically, the failure detection unit 501 a periodically transmits a heart beat to the storage nodes 100 a to 100 d so that it can detect a failure of each of the storage nodes 100 a to 100 d constituting the storage system upon break-off of a corresponding response to the heart beat.

Further, the failure detection unit 501 a can detect a failure of each of the cache memories 1021 a to 1021 d constituting the storage system. When one of the storage nodes 100 a to 100 d detects a failure of corresponding one of the cache memories 1021 a to 1021 d assigned thereto, for example, by detecting the fact that data cannot be written into the particular one of the cache memories 1021 a to 1021 d, the relevant storage node notifies the failure of the assigned cache memory to the control node 500 a. By receiving the notification of the failure of the cache memory, the failure detection unit 501 a detects the failure of the one of the cache memories 1021 a to 1021 d.

More specifically, the failure detection unit 501 a of the control node 500 a communicates with each storage node to periodically monitor whether a write process to each cache memory has succeeded or failed. The failure detection unit 501 a collects via communication the results of monitoring whether the write process to the cache memory of each storage node has succeeded or failed. The failure detection unit 501 a can detect the failure of each of the cache memories 1021 a to 1021 d by checking whether a preset determination condition, e.g., “if the write process to a particular cache memory has failed successively over a predetermined number of times, that cache memory is regarded as being failed”, is satisfied.

Though described in detail later, when the failure of the storage node 100 a is detected by the failure detection unit 501 a, the assignment unit 502 a assigns the storage device 200 a and the cache memory 1021 b assigned so far to the storage node 100 a, for which the failure has been detected by the failure detection unit 501 a, to another storage node. While in this embodiment the assignment unit 502 a assigns the storage device 200 a and the cache memory 1021 b both assigned so far to the storage node 100 a, for which the failure has been detected, to another storage node, the assignment method is not limited to that described in this embodiment. The assignment unit 502 a may assign only one of the storage device 200 a and the cache memory 1021 b to another storage node.

Further, when a cache memory is newly assigned to the storage node 100 a, the assignment unit 502 a assigns, to the storage node 100 a, the cache memory 1021 b which is included in the storage node 100 b other than the storage node 100 a. Stated another way, the cache memory 1021 b is assigned to the storage node (e.g., the storage node 100 a in this embodiment) other than the storage node 100 b which includes the cache memory 1021 b. Therefore, for example, even when the storage node 100 a has failed, the data to be written into the storage device 200 a, which is assigned to the storage node 100 a, is not lost and remains in the cache memory 1021 b which is included in the storage node 100 b. As a result, failure resistance is enhanced and reliability of the storage system is increased.

Thus, when the storage node 100 a has failed, the storage device 200 a and the cache memory 1021 b, which have been so far assigned to the failed storage node 1100 a, are assigned to another storage node by the control node 500 a after it detects the failure of the storage node 100 a.

Further, when the failure of the cache memory 1021 b is detected by the failure detection unit 501 a, the assignment unit 502 a assigns another cache memory to the storage node 100 a to which the cache memory 1021 b has been assigned.

While in this embodiment the assignment of the storage device 200 a and the cache memory 1021 b to the storage node 100 a is preformed by the control node 500 a (or the control node 500 b), the assignment method is not limited to that described in this embodiment. For example, the assignment may be performed by the storage node 100 a itself or another suitable device in an automatic manner according to a particular policy based on, e.g., load balance, or by the administrator in a manual manner as appropriate.

The storage device 200 a and the cache memory 1021 b are assigned by the control node 500 a which controls the storage system by controlling the storage node 100 a.

The terminal device 400 a is a computer operated by the user of the storage system. The user of the storage system accesses the storage node 100 a by operating the terminal device 400 a. In response to a request from the user operating the terminal device 400 a, the storage node 100 a writes and reads data into and from the storage device 200 a.

The operation of the storage node 100 a will be described below with reference to FIG. 4. The operation of writing data into the storage device 200 a is first described. It is here assumed that, at the startup of the storage system, the storage device 200 a and the cache memory 1021 b are assigned to the storage node 100 a by the control node 500 a.

When the terminal device 400 a is operated by the user of the storage system and writing of data is requested to the storage node 100 a, the storage node 100 a writes the data into the storage device 200 a in response to the user's request.

At that time, the backup unit 111 a in the storage node 100 a stores, in the RAM 102 a, the data to be written into the storage device 200 a. Then, the backup unit 111 a starts writing of the data stored in the RAM 102 a into the cache memory 1021 b and the storage device 200 a, which are assigned to the storage node 100 a by the control node 500 a. Herein, because a time required for writing the data into the cache memory 1021 b is shorter than that required for writing the data into the storage device 200 a, the data writing into the cache memory 1021 b is completed earlier.

When the data writing into the storage device 200 a has been normally performed and completed, the backup unit 111 a erases the data that has been written into the RAM 102 a and the cache memory 1021 b. On the other hand, if an abnormality occurs in the data writing into the storage device 200 a and the data writing is not completed, the backup unit 111 a does not erase the data that has been written into the RAM 102 a and the cache memory 1021 b. As a result, even if a failure occurs in any of the storage node 100 a, the cache memory 1021 b, and the storage device 200 a, loss of data can be prevented. Stated another way, when the data is normally written into the storage device 200 a, the data having been written into the cache memory 1021 b by the backup unit 111 a is erased from the cache memory 1021 b by the backup unit 111 a after the completion of the data writing.

Thus, because the data having been completely written into the storage device 200 a is erased from the cache memory 1021 b, the data still stored in the cache memory 1021 b is data that is not yet written into the storage device 200 a. In other words, in the cache memory 1021 b, there is only the data which is to be written into the storage device 200 a in response to the write request from the user, but which is not yet written into the storage device 200 a. Further, the other storage nodes also have the same functions as those of the storage node 100 a. Accordingly, by assigning the storage device 200 a and the cache memory 1021 b to one of the other storage nodes by the control node 500 a, the not-yet-written data remaining in the cache memory 1021 b can be written into the storage device 200 a by the other storage node.

The operation of the take-over unit 112 a in the storage node 100 a will be described below in connection with, for example, the case of a failure generated in a storage node (not shown) to which the cache memory 1021 b has been assigned. When the relevant storage node has failed after data has been written into the cache memory 1021 b by the backup unit (not shown) in the failed storage node, but before the data writing into the storage device 200 a is completed, the writing of the data is not completed and therefore the not-yet-written data remains in the cache memory 1021 b without being erased. Thereafter, the storage device 200 a and the cache memory 1021 b are newly assigned to the storage node 100 a by the control node 500 a. The not-yet-written data remaining in the cache memory 1021 b is read from the cache memory 1021 b by the take-over unit 112 a of the storage node 100 a and is written into the storage device 200 a.

Stated another way, so long as the not-yet-written data which is to be written into the storage device 200 a by the failed storage node and which is not yet written into the storage device 200 a by the failed storage node 100 a remains in the cache memory 1021 b, it is possible to determine the presence or absence of the not-yet-written data and to specify the not-yet-written data itself. The not-yet-written data is written into the storage device 200 a by the take-over unit 112 a of the storage node 100 a which is set to succeed the failed storage node when the storage device 200 a and the cache memory 1021 b are newly assigned.

The take-over unit 112 a and the storage node 100 a can realize the above-mentioned process by various methods. In one example of the various methods, as described above, after a certain storage node has completed the writing of the data received from the terminal device 400 a into the storage device, the data stored in the cache memory 1021 b by the backup unit of the certain storage node is erased. With such erasing, data in the progress of writing into the storage device 200 a, which is set in a corresponding relation to the cache memory 1021 b by assignment, is stored in the cache memory 1021 b, whereas the data having been already reflected is erased. As a result, only the not-yet-reflected data is stored in the cache memory 1021 b. Thereafter, when the storage device 200 a and the cache memory 1021 b are newly assigned, the take-over unit 112 a of the storage node 100 a writes the data stored in the cache memory 1021 b into the storage device 200 a.

In another example, a table is prepared to load the data stored in the storage device 200 a and the cache memory 1021 b, which correspond to each other by assignment. At the time of a new assignment, the table is checked and data stored (updated) in only the cache memory 1021 b is reflected on the storage device 200 a. Other suitable methods are also usable.

The connection state table used in the control nodes 500 a and 500 b in this embodiment will be described below.

FIG. 5 illustrates the data structure of the connection state table. A connection state table 510, shown in FIG. 5, is prepared and managed by the control node 500 a serving as the currently operating system and the control node 500 b maintained as the standby system. The connection state table 510 stores connection information with respect to the storage nodes, the storage devices, and the cache memories in the storage system.

The connection state table 510 has a column of “storage node name” indicating respective names of the storage nodes arranged in the storage system, a column of “storage name” indicating respective names of the storage devices arranged in the storage system, and a column of “cache device name” indicating respective names of the cache memories arranged in the storage system. Items of the information lying horizontally in the three columns are correlated with one another to provide connection state information representing the storage device and the cache memory which are assigned to the storage node.

The connection state information stored in the connection state table 510 is updated by the currently operating control node 500 a, as required, when the storage system is started up and when the assignment is changed due to a failure, etc. The same table is also stored in the standby control node 500 b and is updated as required.

For example, the top row of the connection state table 510 in FIG. 5 stores information indicating that the storage node name is “SN-A”, the storage (device) name is “DISK-A”, and the cache device name is “CACHE-A”.

The above information means that the storage device having the name “DISK-A” and the cache memory having the name “CACHE-A” are assigned to the storage node having the name “SN-A”.

In other words, the above information means that the storage node “SN-A” manages the storage device “DISK-A” and the cache memory “CACHE-A”, and that data written into the storage device “DISK-A” is also written into the cache memory “CACHE-A”.

The operation of the storage system in a process of assigning the storage device and the cache memory will be described below. FIG. 6 illustrates one example of assignment when the storage system is started up. FIG. 7 illustrates one example of assignment when the storage node has failed. FIG. 8 illustrates one example of assignment when the cache memory has failed. The state of the storage system after the assignment at the startup of the storage system will be first described with reference to FIG. 6.

The following description is made by extracting, as shown in FIG. 6, only the related components, i.e., the storage nodes 100 a and 100 b, the storage devices 200 a and 200 b, the terminal device 400 a, the control node 500 a, cache memories 1021 p and 1021 q, and the networks 30 and 40.

For the convenience of explanation, the network 30 and the network 40 are shown as one network in FIG. 6. The illustrated components are interconnected, as required, via the networks 30 and 40. It is also assumed that the cache memories 1021 p and 1021 q are included in not-shown storage nodes other than the storage nodes 100 a and 100 b.

In FIG. 6, at the startup of the storage system, the storage device 200 a and the cache memory 1021 p are assigned to the storage node 100 a by the control node 500 a. With that assignment, the storage node 100 a manages the storage device 200 a and uses the cache memory 1021 p for backup when data is written into the storage device 200 a.

For example, assuming that the user enters a request for writing data into the storage device 200 a by using the terminal device 400 a, upon receiving the data transmitted from the terminal device 400 a, the storage node 100 a temporarily writes the received data in the cache memory 1021 p for backup. Then, the storage node 100 a writes the received data into the storage device 200 a. When the data writing into the storage device 200 a has been normally performed and completed, the storage node 100 a erases the data having been written into the cache memory 1021 p. In such a manner, the storage node 100 a writes data into the storage device 200 a while backing up the data by using the cache memory 1021 p.

Further, at the startup of the storage system, the storage device 200 b and the cache memory 1021 q are assigned to the storage node 100 b by the control node 500 a. When the user enters a data write request by using the terminal device 400 a, the storage node 100 b operates similarly to the storage node 100 a and writes data into the storage device 200 b while backing up the data by using the cache memory 1021 q.

In this embodiment, the storage node is exclusively assigned to the storage device and the cache memory. Stated another way, in FIG. 6, the storage node 100 a to which the storage device 200 a and the cache memory 1021 p are assigned can exclusively accesses the storage device 200 a and the cache memory 1021 p. Any of the other storage nodes cannot access the storage device 200 a and/or the cache memory 1021 p unless it is assigned thereto.

The above-described assignment setting of the storage system is managed by the control node 500 a as the connection state information that is stored in the connection state table shown in FIG. 5.

The state of the storage system after assignment performed in the event of a failure of the storage node 100 a will be described below with reference to FIG. 7.

The following description is made by extracting, as shown in FIG. 7, only the related components, i.e., the storage nodes 100 a to 100 c, the storage devices 200 a and 200 b, the terminal device 400 a, the control node 500 a, the cache memories 1021 p and 1021 q, and the networks 30 and 40.

For the convenience of explanation, as in FIG. 6, the network 30 and the network 40 are shown as one network in FIG. 7. The illustrated components are interconnected, as required, via the networks 30 and 40. It is also assumed that the cache memories 1021 p and 1021 q are included in not-shown storage nodes other than the storage nodes 100 a to 100 c.

In FIG. 7, as in the case of FIG. 6, the storage device 200 b and the cache memory 1021 q are assigned to the storage node 100 b by the control node 500 a. On the other hand, the storage node 100 a has failed. The storage device 200 a and the cache memory 1021 p, which have been so far assigned to the failed storage node 100 a, are newly assigned to the storage node 100 c by the control node 500 a, which has detected the failure of the storage node 100 a. With that reassignment, the storage node 100 c manages the storage device 200 a and uses the cache memory 1021 p for backup when data is written into the storage device 200 a.

For example, when the user enters a request for writing data into the storage device 200 a by using the terminal device 400 a, the newly assigned storage node 100 c receives the data transmitted from the terminal device 400 a and temporarily writes the received data in the cache memory 1021 p for backup. Then, the storage node 100 c writes the received data into the storage device 200 a. When the data writing into the storage device 200 a has been normally performed and completed, the storage node 100 c erases the data having been written into the cache memory 1021 p. In such a manner, the storage node 100 c newly assigned instead of the storage node 100 a writes data into the storage device 200 a while backing up the data by using the cache memory 1021 p.

The state of the storage system after assignment performed in the event of a failure of the cache memory will be described below with reference to FIG. 8.

The following description is made by extracting, as shown in FIG. 8, only the related components, i.e., the storage nodes 100 a and 100 b, the storage devices 200 a and 200 b, the terminal device 400 a, the control node 500 a, the cache memories 1021 p to 1021 r, and the networks 30 and 40.

For the convenience of explanation, as in FIG. 6, the network 30 and the network 40 are shown as one network in FIG. 8. The illustrated components are interconnected, as required, via the networks 30 and 40. It is also assumed that the cache memories 1021 p to 1021 r are included in not-shown storage nodes other than the storage nodes 100 a and 100 b.

In FIG. 8, as in the case of FIG. 6, the storage device 200 a and the cache memory 1021 p are assigned to the storage node 100 a by the control node 500 a. On the other hand, the cache memory 1021 q has failed. The cache memory 1021 r is newly assigned to the storage node 100 b, to which the cache memory 1021 q has been so far assigned, by the control node 500 a, which has detected the failure of the cache memory 1021 q. With that reassignment, the storage node 100 b manages the storage device 200 b and uses the cache memory 1021 r for backup when data is written into the storage device 200 b.

For example, when the user enters a request for writing data into the storage device 200 b by using the terminal device 400 a, the storage node 100 b receives the data transmitted from the terminal device 400 a and temporarily writes the received data in the cache memory 1021 r for backup. Then, the storage node 100 b writes the received data into the storage device 200 b. When the data writing into the storage device 200 b has been normally performed and completed, the storage node 100 b erases the data having been written into the cache memory 1021 r. In such a manner, the storage node 100 b writes data into the storage device 200 b while backing up the data by using the cache memory 1021 r which has been newly assigned instead of the cache memory 1021 q.

While in this embodiment the cache memories are arranged as respective portions of the RAMs in the storage nodes, the arrangement of the cache memories is not limited to that described in this embodiment. For example, the cache memories may be arranged, separately from the storage nodes, as independent cache devices in the form of, e.g., a cache dedicated server or a cache dedicated storage device on the network.

The above-described changes of the assignment setting caused with the occurrence of failures are also reflected by the control node 500 a, as described above with reference to FIG. 6, on update of the connection state information that is stored and managed in the connection state table shown in FIG. 5.

The update of the connection state information with the assignment will be described below. FIGS. 9A and 9B are tables for explaining the update of the connection state table when the storage node has failed, and FIGS. 10A and 10B are tables for explaining the update of the connection state table when the cache memory has failed.

The update of the connection state table with the assignment performed upon the occurrence of a failure of the storage node is first described with reference to FIGS. 9A and 9B. FIG. 9A illustrates the connection state table before a failure occurs in the storage node “SN-A”. FIG. 9B illustrates the connection state table in the updated state after the assignment performed upon the occurrence of the failure of the storage node “SN-A” has been reflected.

As described above, the connection state table is stored in the control nodes 500 a and 500 b for managing the assignment setting in the storage system.

The following description is made in connection with the case where the storage node “SN-A” has failed and the storage device “DISK-A” and the cache memory “CACHE-A”, which have been so far assigned to the storage node “SN-A”, are newly assigned to the storage node “SN-C”. Note that the storage node “SN-A” in FIGS. 9A and 9B corresponds to the storage node 100 a in FIG. 7. Similarly, the storage device “DISK-A” corresponds to the storage device 200 a in FIG. 7 and the cache memory “CACHE-A” corresponds to the cache memory 1021 a in FIG. 7.

The connection state information put in the top row of a connection state table 520, shown in FIG. 9A, indicates that the storage node “SN-A”, the storage device “DISK-A”, and the cache memory “CACHE-A” are correlated with one another. This means that the storage device “DISK-A” and the cache memory “CACHE-A” are assigned to the storage node “SN-A”.

The connection state information put in the top row of a connection state table 530, shown in FIG. 9B, indicates that the storage node “SN-C”, the storage device “DISK-A”, and the cache memory “CACHE-A” are correlated with one another. This means that the storage device “DISK-A” and the cache memory “CACHE-A” are assigned to the storage node “SN-C”.

Stated another way, in FIGS. 9A and 9B, since the storage device “DISK-A” and the cache memory “CACHE-A” are assigned by the control nodes 500 a and 500 b to the storage node “SN-C” instead of the failed storage node “SN-A”, the connection state table is updated from the connection state table 520, shown in FIG. 9A, to the connection state table 530, shown in FIG. 9B, corresponding to the above-described assignment.

The update of the connection state table with the assignment performed upon the occurrence of a failure of the cache memory is next described with reference to FIGS. 10A and 10B. FIG. 10A illustrates the connection state table before a failure occurs in the cache memory “CACHE-A”. FIG. 10B illustrates the connection state table in the updated state after the assignment made upon the occurrence of a failure of the cache memory “CACHE-A” has been reflected.

The following description is made in connection with the case where the cache memory “CACHE-A” assigned to the storage node “SN-A” has failed and the cache memory “CACHE-B” is newly assigned instead of the cache memory “CACHE-A” to the storage node “SN-A”. Note that the storage node “SN-A” in FIGS. 10A and 10B corresponds to the storage node 100 a in FIG. 8. Similarly, the storage device “DISK-A” corresponds to the storage device 200 a in FIG. 8 and the cache memory “CACHE-A” corresponds to the cache memory 1021 a in FIG. 8.

As in the table of FIG. 9A, the connection state information put in the top row of a connection state table 540, shown in FIG. 10A, indicates that the storage node “SN-A”, the storage device “DISK-A”, and the cache memory “CACHE-A” are correlated with one another. This means that the storage device “DISK-A” and the cache memory “CACHE-A” are assigned to the storage node “SN-A”.

The connection state information put in the top row of a connection state table 550, shown in FIG. 10B, indicates that the storage node “SN-A”, the storage device “DISK-A”, and the cache memory “CACHE-B” are correlated with one another. This means that the storage device “DISK-A” and the cache memory “CACHE-B” are assigned to the storage node “SN-A”.

Stated another way, in FIGS. 10A and 10B, since the cache memory “CACHE-B” is assigned instead of the failed cache memory “CACHE-A” to the storage node “SN-A” by the control nodes 500 a and 500 b, the connection state table is updated from the connection state table 540, shown in FIG. 10A, to the connection state table 550, shown in FIG. 10B, corresponding to the above-described assignment.

The procedures of processing executed in the storage system of this embodiment will be described below. A startup assignment process is first described which is executed at the startup of the storage system of this embodiment. FIG. 11 is a sequence chart showing the procedures of the assignment process at the startup of the storage system.

At the startup of the storage system of this embodiment, the storage device and the cache memory are assigned to each storage node by the control node through polling. With the assignment, the control node exclusively connects the storage device and the cache memory to the storage node.

With reference to FIG. 11, a description is made of the case where the storage device (e.g., the storage device 200 a) and the cache memory (e.g., the cache memory 1021 b) are assigned to the storage node (e.g., the storage node 100 a) by the currently operating control node (e.g., the control node 500 a). It is here assumed that the cache memory 1021 b is arranged inside the RAM 102 b which is included in the storage node 100 b.

[Step S110] The control node 500 a notifies the storage node 100 a of the assignment of the storage device 200 a and the cache memory 1021 b thereto via the network 30 and issues a connection request.

[Step S121] Upon receiving the connection request transmitted from the control node 500 a, the storage node 100 a executes a process of stopping reception of access made on the storage device which is assigned to the storage node 100 a. With that process, even if there is the storage device assigned to the storage node 100 a, access to data (i.e., reading/writing of data) stored in the relevant storage device from the terminal devices 400 a to 400 c, etc. is temporarily stopped. When access to the data stored in the storage device 200 a is tried from the terminal devices 400 a to 400 c during a period until the stop of access is canceled and the access is resumed, an error response is transmitted to the terminal device having tried the access.

[Step S122] If not-yet-written data which is to be written into the storage device 200 a, but which is not yet written into the storage device 200 a is present in the cache memory 1021 b, the storage node 100 a executes a process of reading the not-yet-written data.

[Step S133] The not-yet-written data to be written into the storage device 200 a, which is written in the cache memory 1021 b, is transmitted from the cache memory 1021 b to the storage node 100 a. This process is executed by the storage node 100 b which includes the cache memory 1021 b.

[Step S124] The storage node 100 a transmits the not-yet-written data, which has been read (transmitted) in step S133, to the storage device 200 a such that the not-yet-written data is written into the storage device 200 a which is assigned to the storage node 100 a by the control node 500 a.

[Step S145] The storage device 200 a stores (writes), in a HDD included therein, the not-yet-written data transmitted from the storage node 100 a to which the storage device 200 a is assigned. When the data writing is completed, the storage device 200 a transmits a write completion response to the storage node 100 a.

[Step S126] The storage node 100 a erases the not-yet-written data, which has been written into the storage device 200 a in step S124, from the cache memory 1021 b.

[Step S137] When the not-yet-written data in the cache memory 1021 b is completely erased, a data erasure completion response is transmitted to the storage node 100 a. As in step S133, this erasure process is executed by the storage node 100 b which includes the cache memory 1021 b.

[Step S128] Upon receiving the data erasure completion response transmitted in step S137, the storage node 100 a executes a process of resuming the reception of access made on the assigned storage device 200 a. Correspondingly, access to the storage device 200 a from the terminal devices 400 a to 400 c, etc. is resumed. As a result, access to the data stored in the storage device 200 a from the terminal devices 400 a to 400 c, etc. is enabled.

[Step S129] The storage node 100 a transmits a connection completion response to the control node 500 a.

In the storage system of this embodiment, as described above, the control node 500 a assigns the storage device 200 a and the cache memory 1021 b to the storage node 100 a at the startup of the storage system. Also, if there is not-yet-written data which is stored in the cache memory 1021 b, but which is not yet written into the storage device 200 a when the storage device 200 a and the cache memory 1021 b are assigned, the storage node 100 a to which the storage device 200 a and the cache memory 1021 b are assigned reads the not-yet-written data from the cache memory 1021 b and writes it into the storage device 200 a.

While in this embodiment the storage node 100 a reads the not-yet-written data, which has been written into the cache memory 1021 b, from the cache memory 1021 b and writes it into the storage device 200 a, the method of writing the not-yet-written data is not limited to that described in this embodiment. For example, the same data as the not-yet-written data in the cache memory 1021 b may be stored in the RAM 102 a of the storage node 100 a or some other suitable memory, and the storage node 100 a may read the not-yet-written data from the RAM 102 a or the other memory and write it into the storage device 200 a.

An ordinary write process will be described below which is executed in the storage system of this embodiment when data is written in the ordinary mode. FIG. 12 is a sequence chart showing the procedures of the write process executed in the storage node in the ordinary mode of the storage system.

In the storage system of this embodiment, a process of writing data, which is transmitted from the terminal device (e.g., the terminal device 400 a) via the networks 30 and 40, into the storage device (e.g., the storage device 200 a), is usually executed through the procedures described below. It is here assumed that the storage device 200 a and the cache memory 1021 b are assigned to the storage node 100 a.

[Step S250] The terminal device 400 a transmits the data, which is to be written into the storage device 200 a, to the storage node 100 a via the networks 30 and 40.

[Step S221] Upon receiving the data transmitted from the terminal device 400 a to be written into the storage device 200 a, the storage node 10 a writes the received data into the cache memory 1021 b.

[Step S232] When the writing of the data transmitted from the terminal device 400 a into the cache memory 1021 b is completed, a write completion response is transmitted to the storage node 100 a. This process is executed by the storage node 100 b which includes the cache memory 1021 b.

[Step S223] Upon receiving the write completion response in step S232, the storage node 100 a transmits the write completion response to the terminal device 400 a.

[Step S224] The storage node 100 a transmits the data transmitted from the terminal device 400 a to the storage device 200 a for writing the data into the storage device (disk) 200 a.

[Step S245] The storage device 200 a stores (writes), in a HDD included therein, the data transmitted from the terminal device 400 a. When the data writing is completed, the storage device 200 a transmits a write completion response to the storage node 100 a.

[Step S226] The storage node 100 a erases the data, which has been written into the storage device 200 a in step S224, from the cache memory 1021 b.

[Step S237] When the data having been written into the cache memory 1021 b is completely erased, a data erasure completion response is transmitted to the storage node 100 a. As in step S232, this erasure process is executed by the storage node 100 b which includes the cache memory 1021 b.

In the storage system of this embodiment, as described above, when the storage node 100 a writes the data transmitted from the terminal device 400 a into the storage device 200 a, the storage node 100 a stores the data in the cache memory 1021 b as well, which is previously assigned to the storage node 100 a, for backup before the data writing into the storage device 200 a is completed.

Further, when the writing of the data, which is to be written into the storage device 200 a, to the cache memory 1021 b is completed, the storage node 100 a transmits the write completion response to the terminal device 400 a. Accordingly, the data transmitted from the terminal device 400 a is released from the write process on the side including the terminal device 400 a before the data writing into the storage device 200 a is completed. Stated another way, in the ordinary mode, the writing of the data transmitted from the terminal device 400 a into the storage device 200 a is processed in an asynchronous manner. As a result, in the ordinary mode, the data from the terminal device 400 a is immediately stored in the cache memory 1021 b and to the terminal device 400 a without waiting for the completion of the data writing into the storage device 200 a, whereby a quicker response can be achieved in spite of taking a backup.

A synchronous write process in the storage system of this embodiment will be described below which is executed in writing data when the cache memory has no vacancy. FIG. 13 is a sequence chart showing the procedures of the write process when the cache memory has no vacancy.

In the storage system of this embodiment, at the time of writing the data, which is transmitted from the terminal device (e.g., the terminal device 400 a) via the networks 30 and 40, into the storage device (e.g., the storage device 200 a), if the data cannot be written into the cache memory due to, e.g., lack of a vacant capacity of the cache memory (e.g., the cache memory 1021 b), the data write process is executed through the procedures described below. It is here assumed, as in the case of FIG. 12, that the storage device 200 a and the cache memory 1021 b are assigned to the storage node 100 a.

[Step S350] The terminal device 400 a transmits the data, which is to be written into the storage device 200 a, to the storage node 100 a via the networks 30 and 40.

[Step S321] Upon receiving the data transmitted from the terminal device 400 a to be written into the storage device 200 a, the storage node 100 a writes the received data into the cache memory 1021 b.

[Step S332] If the writing of the data transmitted from the terminal device 400 a into the cache memory 1021 b cannot be performed, a write disability response for notifying the disability of the data writing is transmitted to the storage node 100 a. This process is executed by the storage node 100 b which includes the cache memory 1021 b.

[Step S323] Upon receiving the write disability response, the storage node 100 a transmits the data transmitted from the terminal device 400 a to the storage device 200 a for writing the data into the storage device (disk) 200 a.

[Step S344] The storage device 200 a stores (writes), in a HDD included therein, the data transmitted from the terminal device 400 a. When the data writing is completed, the storage device 200 a transmits a write completion response to the storage node 100 a.

[Step S325] Upon receiving the write completion response in step S344, the storage node 100 a transmits the write completion response to the terminal device 400 a.

[Step S326] If there is any data which has been written into the cache memory 1021 b in step S321, the storage node 100 a erases that data from the cache memory 1021 b.

[Step S337] When the data having been written into the cache memory 1021 b is erased in step S326, a data erasure completion response is transmitted to the storage node 100 a upon the completion of the data erasure. As in step S332, this erasure process is executed by the storage node 100 b which includes the cache memory 1021 b.

In this embodiment, as described above, at the time of writing the data, which is transmitted from the terminal device 400 a, into the storage device 200 a, if the data cannot be written into the cache memory 1021 b due to, e.g., deficiency of its vacant capacity, the storage node 100 a directly writes the data into the storage device 200 a without writing it into the cache memory 1021 b. In other words, when the data cannot be backed up in the cache memory 1021 b, the storage node 100 a writes the data into the storage device 200 a in a synchronous manner.

An assignment process in the event of a failure of the storage node will be described below which is executed as an assignment process in the storage system of this embodiment when the storage node has failed. FIG. 14 is a sequence chart showing the procedures of the assignment process executed when the storage node has failed.

In the event of a failure of the storage node in this embodiment, when the failure of the storage node is detected by the control node, the storage device and the cache memory which have been assigned to the failed storage node are assigned to the alternative storage node. Stated another way, the control node reassigns the storage device and the cache memory to the normal storage node. Correspondingly, the normal storage node is exclusively connected to the reassigned storage device and cache memory.

With reference to FIG. 14, the following description is made in connection with the case that, when the storage node (e.g., the storage node 100 c) has failed, the storage device (e.g., the storage device 200 a) and the cache memory (e.g., the cache memory 1021 b), which have been so far assigned to the failed storage node 100 c, are assigned by the currently operating control node (e.g., the control node 500 a) to, instead of the failed storage node 100 c, the storage node (e.g., the storage node 100 a) to which another not-shown storage device and another not-shown cache memory are assigned. It is here assumed that the cache memory 1021 b is arranged in the RAM 102 b which is included in the storage node 100 b. Further, it is assumed that the storage node 100 a to be substituted for the failed storage node 100 c is already operated in the storage system and is assigned to the other storage device and the other cache memory.

Procedures from step S411 (connection request) to step S462 (connection completion response) in the assignment process in the event of a failure of the storage node are the same as those in the startup assignment process in FIG. 11. Therefore, the illustration and the description regarding details of the procedures from step S411 to step S462 are omitted here.

[Step S410] The control node 500 a detects a failure of the storage node 100 c. The control node 500 a periodically transmits a heart beat so that it can detect a failure of any of the storage nodes constituting the storage system upon break-off of a corresponding response to the heart beat.

[Step S411] Upon detecting the failure of the storage node 100 c in step S410, the control node 500 a notifies, via the network 30, the storage node 100 a of the fact that the storage device 200 a and the cache memory 1021 b are now assigned to the storage node 100 a which is substituted for the failed storage node 100 c, and issues a connection request for the corresponding connection. Then, the startup assignment process is executed as described above with reference to FIG. 11.

[Step S462] When the connection of the assigned storage device 200 a and cache memory 1021 b to the storage node 100 a is completed, the storage node 100 a transmits a connection completion response to the control node 500 a.

In this embodiment, as described above, the storage node 100 a to be substituted for the failed storage node 100 c is assigned to the other storage device and the other cache memory. In other words, the storage device 200 a and the cache memory 1021 b are further assigned to the storage node 100 a which is already assigned to the other storage device and the other cache memory. However, the assignment method is not limited to that described in this embodiment. A standby storage node may be arranged in the storage system so that, in the event of a failure of the currently used storage node, the storage device and the cache memory which have been so far assigned to the failed storage node may be assigned to the standby storage node.

Further, one of the storage nodes constituting the storage system of this embodiment, which has a minimum load, may be automatically selected by the control node 500 a as the storage node 100 a to be substituted for the failed storage node 100 c. In that case, for example, the frequency of data reading/writing may be used as a parameter for evaluating the load of each storage node. The frequency of data reading/writing may be represented by the number of accesses or by the amount of data having been read/written during the latest certain time. As another example of the parameter for evaluating the load of each storage node, the magnitude of capacity of the assigned storage device may also be used.

In the storage system of this embodiment, as described above, in the event of a failure of the storage node 100 c, when the control node 500 a detects the failure of the storage node 100 c, the control node 500 a newly assigns the storage device 200 a and the cache memory 1021 b to the storage node 100 a. Also, if there is not-yet-written data which is stored in the cache memory 1021 b, but which is not yet written into the storage device 200 a at the timing of the assignment of the storage device 200 a and the cache memory 1021 b to the storage node 100 a, the storage node 100 a to which the storage device 200 a and the cache memory 1021 b are newly assigned reads the not-yet-written data from the cache memory 1021 b and writes it into the storage device 200 a (see FIG. 11).

An assignment process in the event of a failure of the cache memory will be described below which is executed as an assignment process in the storage system of this embodiment when the cache memory has failed. FIG. 15 is a sequence chart showing the procedures of the assignment process executed when the cache memory has failed.

In the event of a failure of the cache memory in the storage system of this embodiment, when the failure of the cache memory is detected by the control node, an alternative cache memory is assigned to the storage node, to which the failed cache memory has been so far assigned, along with the storage device which has been so far assigned to the same storage node. In other words, the control node exclusively assigns the storage device and the normal cache memory to the storage node to which the failed cache memory has been so far assigned. Correspondingly, that storage node is connected to the assigned normal cache memory.

With reference to FIG. 15, the following description is made in connection with the case that, when some cache memory (e.g., the cache memory 1021 c) has failed, another normal cache memory (e.g., the cache memory 1021 b) is assigned, instead of the failed cache memory, to the storage node (e.g., the storage node 100 a) to which the failed cache memory has been so far assigned, in addition to the storage device (e.g., the storage device 200 a) which is already assigned to the relevant storage node. It is here assumed that the cache memory 1021 b is arranged in the RAM 102 b which is included in the storage node 100 b. Similarly, the cache memory 1021 c is arranged in the RAM 102 c which is included in the storage node 10 c. Further, it is assumed that the cache memory 1021 b is already operated in the storage system and is assigned to another not-shown storage node along with another not-shown storage device.

Procedures from step S511 (ordinary-to-synchronous mode change instruction) to step S522 (ordinary-to-synchronous mode change response) (i.e., cancellation of assignment of the failed cache memory 1021 c) in the assignment process in the event of a failure of the cache memory will be described in detail later as an ordinary-to-synchronous mode change process with reference to FIG. 16. Also, procedures from step S513 (synchronous-to-ordinary mode change instruction) to step S524 (synchronous-to-ordinary mode change response) (i.e., new assignment of the normal cache memory 1021 b) will be described in detail later as a synchronous-to-ordinary mode change process with reference to FIG. 18.

[Step S510] The control node 500 a detects a failure of the cache memory 1021 c. More specifically, the control node 500 a communicates with each storage node to periodically monitor whether a write process to each cache memory has succeeded or failed. The control node 500 a collects, via communication, the results of monitoring whether the write process from each storage node to the corresponding cache memory has succeeded or failed. The control node 500 a can detect the failure of each of the cache memories constituting the storage system by checking whether a preset determination condition, e.g., “if the write process to a particular cache memory has failed successively over a predetermined number of times, that cache memory is regarded as being failed”, is satisfied.

[Step S511] Upon detecting the failure of the cache memory 1021 c in step S510, the control node 500 a transmits an ordinary-to-synchronous mode change instruction to the storage node 100 a, to which the failed cache memory 1021 c has been so far assigned, for changing from the ordinary mode to the synchronous mode in which data is written without using the cache memory. The change from the ordinary mode to the synchronous mode will be described in detail later as the ordinary-to-synchronous mode change process with reference to FIG. 16.

[Step S522] When the ordinary-to-synchronous mode change process (see FIG. 16) is completed, the storage node 100 a transmits an ordinary-to-synchronous mode change response to the control node 500 a.

[Step S513] Upon receiving the ordinary-to-synchronous mode change response in step S522, the control node 500 a transmits a synchronous-to-ordinary mode change instruction to the storage node 100 a, to which the failed cache memory 1021 c has been so far assigned, for changing from the synchronous mode to the ordinary mode in which data is written by using the normal cache memory 1021 b for backup. A process of performing the change from the synchronous mode to the ordinary mode will be described in detail later as the synchronous-to-ordinary mode change process with reference to FIG. 18.

[Step S524] When the synchronous-to-ordinary mode change process (see FIG. 18) is completed, the storage node 100 a transmits a synchronous-to-ordinary mode change response to the control node 500 a.

In this embodiment, as described above, it is assumed that the cache memory 1021 b is already operative in the storage system and is assigned to another not-shown storage node along with another not-shown storage device. In other words, the cache memory 1021 b which has already been assigned to the other not-shown storage node is further assigned to the storage node 100 a. However, the assignment method is not limited to that described in this embodiment. A standby cache memory may be arranged in the storage system so that, in the event of a failure of the currently used cache memory, the standby cache memory may be assigned to the storage node to which the failed cache memory has been so far assigned.

Alternatively, one of the cache memories constituting the storage system, which has a minimum load, may be automatically selected by the control node 500 a as the cache memory 1021 a to be substituted for the failed cache memory 1021 c. In that case, for example, the frequency of data reading/writing with respect to the cache memory may be used as a parameter for evaluating the load of each cache memory. The frequency of data reading/writing may be represented by the number of times of accesses or by the amount of data having been read/written during the latest certain time. As another example of the parameter for evaluating the load of each cache memory, an average value of vacant capacity of the cache memory during a certain time may also be used.

In the storage system of this embodiment, as described above, when the control node 500 a detects a failure of the cache memory 1021 c, the control node 500 a transmits the ordinary-to-synchronous mode change instruction to the storage node 100 a, to which the failed cache memory 1021 c has been so far assigned. Upon receiving the ordinary-to-synchronous mode change instruction, the storage node 100 a executes the ordinary-to-synchronous mode change process (see FIG. 16) for changing from the ordinary mode, which requires the cache memory in writing data into the storage device 200 a, to the synchronous mode, which does not require the cache memory in writing data into the storage device 200 a, such that the data can be written without using the failed cache memory 1021 c.

Then, the control node 500 a newly assigns the cache memory 1021 b to the storage node 100 a and transmits the synchronous-to-ordinary mode change instruction to the storage node 100 a. When the storage node 100 a is assigned with the cache memory 1021 b and receives the synchronous-to-ordinary mode change instruction, the storage node 100 a executes, subsequent to the completion of the ordinary-to-synchronous mode change process, the synchronous-to-ordinary mode change process (see FIG. 18) for changing from the synchronous mode to the ordinary mode, which employs the newly assigned cache memory 1021 b, such that data can be written while the data is backed up in the newly assigned cache memory 1021 b.

The ordinary-to-synchronous mode change process will be described below. This process is executed in the storage system of this embodiment as a process of performing the change from the ordinary mode, which requires the cache memory in writing data into the storage device, to the synchronous mode, which does not require the cache memory in writing data into the storage device. FIG. 16 is a sequence chart showing the procedures of the process of performing the change from the ordinary mode to the synchronous mode.

In the storage system of this embodiment, in the event of a failure of the cache memory, when the failure of the cache memory is detected by the control node, the ordinary-to-synchronous mode change instruction is transmitted from the control node to the storage node, to which the failed cache memory has been so far assigned, for disconnection of the failed cache memory from the relevant storage node and for changing to the synchronous mode which does not require the cache memory in writing data into the storage device. Upon receiving the ordinary-to-synchronous mode change instruction, the storage node executes a process of writing data into the storage device without using the failed cache memory.

With reference to FIG. 16, the following description is made in connection with the case where the currently operating control node (e.g., the control node 500 a) cancels, for the storage node (e.g., the storage node 100 a) to which the failed cache memory (e.g., the cache memory 1021 c) has been so far assigned, the assignment of the failed cache memory 1021 c thereto and changes the operation mode to the synchronous mode in which data is written into the storage device 200 a without using the cache memory.

It is here assumed that the cache memory 1021 b is arranged in the RAM 102 b which is included in the storage node 100 b. Similarly, the cache memory 1021 c is arranged in the RAM 102 c which is included in the storage node 100 c. Further, it is assumed that the cache memory 1021 b is already operating in the storage system and is assigned to another not-shown storage node along with another not-shown storage device. In addition, it is assumed that the cache memory 1021 c is assigned to the storage node 100 a in a corresponding relation to the storage device 200 a until the cache memory 1021 c fails, and data to be written into the storage device 200 a is written into the cache memory 1021 c for backup until the writing of the data into the storage device 200 a is completed.

[Step S610] Upon detecting the failure of the cache memory 1021 c in step S510 of FIG. 15, the control node 500 a transmits, as in step S511 of FIG. 15, the ordinary-to-synchronous mode change instruction to the storage node 100 a, to which the failed cache memory 1021 c has been so far assigned, for changing to the synchronous mode in which data is written without using the cache memory.

[Step S621] Upon receiving the ordinary-to-synchronous mode change instruction transmitted from the control node 500 a, the storage node 100 a executes a process of stopping reception of access made on the storage device 200 a which is assigned to the storage node 100 a along with the failed cache memory 1021 c. In other words, that process temporarily stops access, from the terminal device 400 a to 400 c, etc., to data (i.e., reading/writing of data) stored in the storage device 200 a which is set in the corresponding relation to the cache memory 1021 c and is assigned to the storage node 100 a. When access to the data stored in the storage device 200 a is tried from the terminal devices 400 a to 400 c during a period until the stop of access is canceled and the access is resumed, an error response is transmitted to the terminal device that tried the access.

[Step S622] If data to be written into the storage device 200 a is stored in the RAM 102 a arranged in the storage node 100 a, the storage node 100 a transmits all the data stored in the RAM 102 a to the storage device 200 a to be directly written into the storage device 200 a without writing it into the cache memory.

[Step S643] The storage device 200 a stores (writes), in the HDD included therein, the data transmitted from the storage node 100 a in step S622. When the data writing is completed, the storage device 200 a transmits a write-of-all-received-data completion response to the storage node 100 a.

[Step S624] Upon receiving the write-of-all-received-data completion response in step S643, the storage node 100 a executes a process of resuming the reception of access made on the storage device 200 a. Correspondingly, access to the storage device 200 a from the terminal devices 400 a to 400 c, etc. is resumed. As a result, access to the data stored in the storage device 200 a from the terminal devices 400 a to 400 c, etc. is enabled. After that point in time, data transmitted from the terminal devices 400 a to 400 c to be written into the storage device 200 a is written in the synchronous mode. If there is no written data in step S622, the process of resuming the reception of access is also executed in a similar manner.

[Step S625] If any data remains in the cache memory 1021 c, the storage node 100 a executes a cache erasure process of erasing the remaining data. When access to the cache memory 1021 c is not allowed or the data in the cache memory 1021 c cannot be erased depending on a failed condition, for example, the cache erasure process of erasing the data is not executed.

[Step S636] When the data having remained in the cache memory 1021 c is completely erased, a cache erasure completion response is transmitted to the storage node 100 a. This erasure process is executed by the storage node 100 c which includes the cache memory 1021 c.

[Step S627] When the above-described process of performing the change from the ordinary mode to the synchronous mode is completed, the storage node 100 a transmits the ordinary-to-synchronous mode change response to the control node 500 a.

In the storage system of this embodiment, as described above, the operation mode of the storage node 100 a is temporarily changed to the synchronous mode through the ordinary-to-synchronous mode change process of performing the change from the ordinary mode, which requires the cache memory in writing data into the storage device 200 a, to the synchronous mode which does not require the cache memory in writing data into the storage device 200 a. As a result, the data is synchronously directly written into the storage device 200 a according to a synchronous write process, which is described later with reference to FIG. 17, without using the failed cache memory 1021 c.

The synchronous write process which will be described below is executed in the storage system of this embodiment as a process in which the storage node directly writes data transmitted from the terminal device into the storage device without writing it in the cache memory for backup. FIG. 17 is a sequence chart showing the procedures of the synchronous write process executed by the storage node.

In the storage system of this embodiment, if the cache memory fails, the operation mode of the storage node is changed to the synchronous mode which does not require the cache memory in writing data into the storage device. In the synchronous mode, the storage node directly writes the data transmitted from the terminal device into the storage device without using the cache memory.

As illustrated in FIG. 17, the process of directly writing data transmitted from the terminal device (e.g., the terminal device 400 a) via the networks 30 and 40 into the storage device (e.g., the storage device 200 a) is executed through procedures described below. It is here assumed that the cache memory 1021 b is assigned to the storage node 100 a along with the storage device 200 a.

[Step S750] The terminal device 400 a transmits the data, which is to be written into the storage device 200 a, to the storage node 10 a via the networks 30 and 40.

[Step S721] Upon receiving the data transmitted from the terminal device 400 a to be written into the storage device 200 a, the storage node 100 a transmits the received data to the storage device 200 a for directly writing it into the storage device (disk) 200 a.

[Step S742] The storage device 200 a stores (writes), in the HDD included therein, the data transmitted from the terminal device 400 a. When the data writing is completed, the storage device 200 a transmits a write completion response to the storage node 100 a.

[Step S723] Upon receiving the write completion response in step S742, the storage node 100 a transmits the write completion response to the terminal device 400 a.

In this embodiment, as described above, when the storage node 100 a writes the data transmitted from the terminal device 400 a into the storage device 200 a in the synchronous mode, the data is directly written into the storage device 200 a without using the cache memory.

Further, when the writing of the data, which has been transmitted from the terminal device 400 a to the storage device 200 a, is completed, the storage node 100 a transmits the write completion response to the terminal device 400 a. Accordingly, the data transmitted from the terminal device 400 a is released from the write process on the side including the terminal device 400 a after the data writing into the storage device 200 a is completed. Stated another way, in the synchronous mode, the writing of the data transmitted from the terminal device 400 a into the storage device 200 a is processed in a synchronous manner.

The synchronous-to-ordinary mode change process which will be described below is executed in the storage system of this embodiment as a process of performing the change from the synchronous mode, in which data is not backed up when the data is written into the storage device, to the ordinary mode, in which data is backed up in the cache memory when the data is written into the storage device. FIG. 18 is a sequence chart showing the procedures of the process of performing the change from synchronous mode to the ordinary mode.

In the storage system of this embodiment, in the event of a failure of the cache memory, when the failure of the cache memory is detected by the control node, the operation mode is temporarily changed to the synchronous mode which does not require the cache memory in writing data into the storage device, and the storage node executes the process of writing the data into the storage device without using the failed cache memory. Then, the control node assigns a new normal cache memory other than the failed cache memory to the storage node to which the failed cache memory has been so far assigned. Subsequently, the control node transmits the synchronous-to-ordinary mode change instruction to the storage node to which the new normal cache memory is assigned, for changing to the ordinary mode again in which the data to be written into the storage device is backed up by using the cache memory. Upon receiving the synchronous-to-ordinary mode change instruction, the storage node executes the process of writing the data into the storage device while backing it up in the normal cache memory newly assigned.

With reference to FIG. 18, the following description is made in connection with the following case. When the currently operating control node (e.g., the control node 500 a) assigns, to the storage node (e.g., the storage node 100 a) to which the failed cache memory (e.g., the cache memory 1021 c) has been so far assigned, another normal cache memory (e.g., the cache memory 1021 b) to be substituted for the failed cache memory 1021 c, the operation mode is changed to the ordinary mode in which data is written into the storage device 200 a while the data is backed up in the newly assigned cache memory 1021 b.

It is here assumed that the cache memory 1021 b is arranged in the RAM 102 b which is included in the storage node 100 b. Similarly, the cache memory 1021 c is arranged in the RAM 102 c which is included in the storage node 100 c. Further, it is assumed that the cache memory 1021 b is already operated in the storage system and is assigned to another not-shown storage node along with another not-shown storage device. In addition, it is assumed that the cache memory 1021 c is assigned to the storage node 100 a in a corresponding relation to the storage device 200 a until the cache memory 1021 c fails, and data to be written into the storage device 200 a is written into the cache memory 1021 c for backup until the writing of the data into the storage device 200 a is completed.

[Step S810] Upon receiving the ordinary-to-synchronous mode change response in step S522 of FIG. 15, the control node 500 a transmits, as in step S513 of FIG. 15, the synchronous-to-ordinary mode change instruction to the storage node 100 a, to which the failed cache memory 1021 c has been so far assigned, for changing to the ordinary mode again in which data is written while the normal cache memory 1021 b is used to back up the data.

[Step S821] Upon receiving the synchronous-to-ordinary mode change instruction transmitted from the control node 500 a, the storage node 100 a executes a process of stopping reception of access made on the storage device 200 a into which data has been written in the synchronous mode. With that process, access to data (i.e., reading/writing of data) stored in the storage device 200 a, which is going to be changed to the ordinary mode, from the terminal devices 400 a to 400 c, etc. is temporarily stopped. When access to the data stored in the storage device 200 a is tried from the terminal devices 400 a to 400 c before the stop of access is canceled and access is resumed, an error response is transmitted to the terminal device having tried the access.

[Step S822] If data to be written into the storage device 200 a is stored in the RAM 102 a arranged in the storage node 100 a, the storage node 100 a transmits all the data stored in the RAM 102 a to the storage device 200 a to be directly written into the storage device 200 a without writing it into the cache memory.

[Step S843] The storage device 200 a stores (writes), in the HDD included therein, the data transmitted from the storage node 100 a in step S822. When the data writing is completed, the storage device 200 a transmits a write-of-all-received-data completion response to the storage node 100 a.

[Step S824] Upon receiving the write-of-all-received-data completion response in step S843, the storage node 100 a executes a process of resuming the reception of access made on the storage device 200 a. Correspondingly, access to the storage device 200 a from the terminal devices 400 a to 400 c, etc. is resumed. As a result, the access to the data stored in the storage device 200 a from the terminal devices 400 a to 400 c, etc. is enabled. After that point in time, data transmitted from the terminal devices 400 a to 400 c to be written into the storage device 200 a is written in the ordinary mode. If there is no written data in step S822, the process of resuming the reception of access is also executed in a similar manner.

[Step S825] When the above-described process of performing the change from the synchronous mode to the ordinary mode is completed, the storage node 100 a transmits a synchronous-to-ordinary mode change response to the control node 500 a.

Thus, when the control node 500 a detects the failure of the cache memory 1021 c, it transmits the ordinary-to-synchronous mode change instruction to the storage node 100 a for instructing the change to the synchronous mode. When the control node 500 a receives the ordinary-to-synchronous mode change response indicating that the change to the synchronous mode in the storage node 100 a has been completed, it newly assigns the normal cache memory 1021 b to the storage node 100 a and transmits the synchronous-to-ordinary mode change instruction to the storage node 100 a for instructing the change to the ordinary mode.

Thus, when the storage node 100 a receives the synchronous-to-ordinary mode change instruction transmitted from the control node 500 a, it responsively changes the operation mode to the ordinary mode in which data is stored in the cache memory 1021 b when the data is written into the storage device 200 a. On the other hand, when the storage node 100 a receives the ordinary-to-synchronous mode change instruction transmitted from the control node 500 a, it responsively changes the operation mode to the synchronous mode in which data is written into the storage device 200 a without storing the data in any cache memory.

In the storage system of this embodiment, as described above, the operation mode of the storage node 100 a is changed to the ordinary mode again through the synchronous-to-ordinary mode change process of performing the change from the synchronous mode in which data is not backed up in any cache memory in writing the data into the storage device, to the ordinary mode in which data is backed up in the cache memory in writing the data into the storage device. As a result, the data is asynchronously written into the storage device through the ordinary write process, shown in FIG. 12, while the data is backed up in the newly assigned cache memory 1021 b.

A practical example of the configuration of the storage system according to this embodiment will be described below. FIG. 19 is a block diagram showing an exemplary configuration of the storage system according to the embodiment. In the exemplary configuration of the storage system shown in FIG. 19, the storage node includes the cache memory for temporarily storing data to be written into the storage device, and the storage device is constituted by a disk array device.

The storage system according to this embodiment, which has the exemplary configuration shown in FIG. 19, includes a plurality (e.g., four) of storage nodes 100 a, 100 b, 100 c and 100 d, a plurality (e.g., four) of storage devices 200 a, 200 b, 200 c and 200 d, at least one control node, e.g., two control nodes 500 a and 500 b, and at least one (e.g., one) terminal device 400 a, which are interconnected via a network 50 through IP switches 700 a and 700 b.

The storage nodes 100 a to 100 d, the control nodes 500 a and 500 b, and the terminal device 400 a are constituted by computers which are interconnected via the network 50 based on IP (Internet Protocol).

The storage nodes 100 a to 100 d, the control nodes 500 a and 500 b, the terminal device 400 a, and the storage devices 200 a to 200 d have each two network interfaces (not shown). Those two network interfaces are connected to the IP switches 700 a and 700 b so that each path is constituted in dual redundancy.

The storage devices 200 a to 200 d are connected respectively to the storage nodes 100 a to 100 d via the network 50 by using iSCSI (Internet Small Computer System Interface). The storage nodes 100 a to 100 d manage data stored in respective logical units in the storage devices 200 a to 200 d which are assigned by the control nodes 500 a and 500 b, and further provide the data under the management to the terminal device 400 a via the network 50.

In addition, the storage nodes 100 a to 100 d provide, to the terminal device 400 a, information processing service utilizing the data stored in the storage devices 200 a to 200 d which are managed respectively by the storage nodes 100 a to 100 d. Stated another way, each of the storage nodes 100 a to 100 d executes a predetermined program in response to a request transmitted from the terminal device 400 a via the network 50 and then executes reading/writing of data and transmission/reception of data in response to requests from the terminal device 400 a.

More specifically, upon receiving a data read request from the terminal device 400 a via the network 50, each of the storage nodes 100 a to 100 d reads the requested data from one of the storage devices 200 a to 200 d which is assigned to the relevant storage node, and then makes a response to the terminal device 400 a having transmitted the read request. Also, upon receiving a data write request from the terminal device 400 a via the network 50, each of the storage nodes 100 a to 100 d writes the data into one of the storage devices 200 a to 200 d which is assigned to the relevant storage node. When the writing of the data is completed, each of the storage nodes 100 a to 100 d makes a response to the terminal device 400 a for indicating the completion of the data writing.

The storage nodes 100 a to 100 d include cache memories 1021 a to 1021 d, respectively. Further, the storage nodes 100 a to 100 d include respective cache servers (not shown) capable of writing, reading, and erasing data transmitted from the terminal device 400 a with respect to the cache memories 1021 a to 1021 d.

Each of the cache servers causes corresponding storage nodes 100 a to 100 d to transmit and receive data to and from another one of the storage nodes 100 a to 100 d to which the cache memory included in the former one storage node is assigned, to write the received data into the cache memory which is included in the former one storage node, and to transmit data read out from the cache memory which is included in the former one storage node.

The cache memories 1021 a to 1021 d are assigned to the storage nodes 100 a to 100 d by the control nodes 500 a and 500 b. On that occasion, each of the cache memories 1021 a to 1021 d is assigned to one of the storage nodes 100 a to 100 d other than the storage node including the relevant cache memory.

For example, the cache memory 1021 a is not assigned to the storage node 100 a including the cache memory 1021 a, and it is assigned to one of the storage nodes 100 b to 100 d other than the storage node 100 a. Speaking conversely, the cache memory other than the cache memory 1021 a, i.e., one of the cache memories 1021 b to 1021 d which is included in one of the storage nodes 100 b to 100 d other than the storage node 100 a, is assigned to the storage node 100 a.

Each of the cache memories 1021 a to 1021 d stores, for backup, data that is transmitted from assigned one of the storage nodes and is written into corresponding one of the storage devices 200 a to 200 d.

With the arrangement described above, for example, when the storage device 200 a and the cache memory 1021 b are assigned to the storage node 100 a, data loss can be avoided even if the storage node 100 a fails. Stated another way, by completely writing, into the cache memory 1021 b, data which is already transmitted from the terminal device 400 a to the storage node 100 a, but which is not yet completely written into the storage device 200 a, the data is not lost even with the failure of the storage node 100 a.

In comparison with the case where the cache memory is included in the storage node which executes the process of writing data into the relevant cache memory, therefore, an advantage can be obtained in avoiding data loss caused by a failure of the relevant storage node.

The control nodes 500 a and 500 b are computers for managing the storage nodes 100 a to 100 d and the storage devices 200 a to 200 d. The control nodes 500 a and 500 b in this embodiment assign respective logical units to the storage nodes 100 a to 100 d. Further, the control nodes 500 a and 500 b assign, to one of the storage nodes 100 a to 100 d, one of the cache memories 1021 a to 1021 d which is included in another one of the storage nodes 100 a to 100 d except for the former one.

In addition, the control nodes 500 a and 500 b acquire information regarding data management from the storage nodes 100 a to 100 d and update the information as required.

For example, the control nodes 500 a and 500 b hold, in the form of a connection state table, the setting of assignment of the storage devices 200 a to 200 d and the cache memories 1021 a to 1021 d to the storage nodes 100 a to 100 d. When the assignment is set at the startup, or when the assignment is changed with, e.g., reassignment upon detection of a failure, the control nodes 500 a and 500 b update the connection state table and record the latest assignment setting therein.

To increase reliability, the control nodes 500 a and 500 b are constituted in dual redundancy and are interconnected to mutually confirm their live states by using a mutual monitoring system such that one control node can detect an abnormality, e.g., a failure, of the other control node. Further, one (e.g., 500 a) of the control nodes 500 a and 500 b is a currently operating system and is operated in the usual state, and the other (e.g., the control node 500 b) is a standby system and is held standby in the usual state. For example, when the control node 500 a serving as the currently operating system has failed, the control node 500 b maintained as the standby system is operated instead of the control node 500 a to succeed the processing executed by the control node 500 a.

When the storage system is started up, the control nodes 500 a and 500 b assign the storage devices 200 a to 200 d and the cache memories 1021 a to 1021 d to the storage nodes 100 a to 100 d.

Further, the control nodes 500 a and 500 b can detect a failure of the storage nodes 100 a to 100 d and a failure of the cache memories. Upon detecting a failure of the storage nodes 100 a to 100 d, the control nodes 500 a and 500 b newly assign the storage device and the cache memory, which have been so far assigned to the failed storage node, to another storage node.

The storage devices 200 a to 200 d are each a disk array device which is constituted by a plurality of HDDs based on RAID5, and they have respective logical units (LUs). The storage nodes to which the logical units of the storage devices 200 a to 200 d are respectively assigned can exclusively access the assigned logical units.

Only the storage node assigned by the control nodes 500 a and 500 b can exclusively access corresponding storage devices 200 a to 200 d via the network 50.

The storage devices 200 a to 200 d are not limited to RAID5 and they may be constituted by using another type of RAID. As an alternative, a disk array of the storage device may be constituted by using another suitable technique other than RAID. Further, the storage devices 200 a to 200 d may be each constituted by a single hard disk without using a disk array, or another suitable storage device.

The terminal device 400 a is a computer operated by a user of the storage system. The user of the storage system can access the storage nodes 100 a to 100 d and can perform reading/writing of data stored in the storage devices 200 a to 200 d by operating the terminal device 400 a.

Assignment settings in the exemplary configuration of the storage system will be described below which are employed in the usual state and when the storage node has failed. FIG. 20 illustrates the assignment setting in the exemplary configuration of the storage system according to the embodiment in the usual state. FIG. 21 illustrates the assignment setting in the exemplary configuration of the storage system according to the embodiment when the storage node has failed.

As shown in FIGS. 20 and 21, the storage nodes 100 a to 100 d include respective cache servers operating with the cache memories 1021 a to 1021 d. The storage device 200 a includes logical units 211 a, 212 a, 213 a and 214 a (also indicated by A-1 to A-4 in the drawings). Similarly, the storage device 200 b includes logical units 211 b to 214 b (also indicated by B-1 to B-4 in the drawings), and the storage device 200 c includes logical units 211 c to 214 c (also indicated by C-1 to C-4 in the drawings).

In FIG. 20, the storage node 100 a is connected to the cache memory 1021 b included in the cache server of the storage node 100 b, to the logical unit 211 a of the storage device 200 a, and to the logical unit 212 b of the storage device 200 b. Also, the storage node 100 c is connected to the cache memory 1021 d included in the cache server of the storage node 100 d, to the logical unit 214 b of the storage device 200 b, and to the logical unit 211 c of the storage device 200 c.

In the event of a failure of the storage node 10 a, as shown in FIG. 21, the cache memory 1021 b, the logical unit 211 a of the storage device 200 a, and the logical unit 212 b of the storage device 200 b, which have been so far assigned to the storage node 100 a, are assigned to, e.g., the storage node 100 c by the currently operating control node 500 a. With such assignment, when the terminal device 400 a accesses, for example, data stored in the logical unit 212 b of the storage device 200 b, the storage node 100 c succeeds the processing having been executed by the storage node 100 a and executes the process of writing and reading data into and from the storage device 200 b. The above description is similarly applied to the logical unit 211 a.

Change of the connection state in the exemplary configuration of the storage system will be described below by referring to connection state tables. FIGS. 22A and 22B illustrate respective connection state tables in the exemplary configuration of the storage system according to the embodiment when the storage system is in the usual state and when the storage node has failed.

A connection state table 560 shown in FIG. 22A and a connection state table 570 shown in FIG. 22B are each a table storing connection state information that indicates the connection state of the storage nodes, the storage devices, and the cache memories in the storage system. The connection state tables 560 and 570 are prepared and managed by the control node 500 a serving as the currently operating system and the control node 500 b maintained as the standby system. The connection state information stored in the connection state tables 560 and 570 is updated by the currently operating control node 500 a, as required, when the storage system is started up and when the assignment is changed due to a failure, etc. The same table is also stored in the standby control node 500 b and is updated as required. In FIGS. 22A and 22B, “SN-A” to “SN-C” denote the storage nodes 100 a to 100 c, respectively. Also, “LU-A-1” denotes the logical unit 211 a. Similarly, “LU-B-2” denotes the logical unit 212 b, “LU-B-4” denotes the logical unit 214 b, and “LU-C-1” denotes the logical unit 211 c.

Each of the connection state tables 560 and 570 has a column of “storage node name” indicating respective names of the storage nodes arranged in the storage system, a column of “storage (logical unit) name” indicating respective names of the logical units of the storage devices arranged in the storage system, and a column of “cache device name” indicating respective names of the cache memories arranged in the storage system. Items of the information lying horizontally in the three columns are correlated with one another to provide the connection state information representing the logical unit and the cache memory which are assigned to the storage node.

For example, the top row of the connection state table 560 in FIG. 22A indicates that the logical unit 211 a (logical unit name “LU-A-1”) and the cache memory 1021 b (cache device name “CACHE-B”) are assigned to the storage node 100 a (storage node name “SN-A”). Similarly, the second top row of the connection state table 560 indicates that the logical unit 212 b (logical unit name “LU-B-2”) and the cache memory 1021 b (cache device name “CACHE-B”) are assigned to the storage node 100 a (storage node name “SN-A”).

Also, the top row of the connection state table 570 in FIG. 22B indicates that the logical unit 211 a (logical unit name “LU-A-1”) and the cache memory 1021 b (cache device name “CACHE-B”) are assigned to the storage node 100 c (storage node name “SN-C”). Similarly, the second top row of the connection state table 570 of FIG. 22 indicates that the logical unit 212 b (logical unit name “LU-B-2”) and the cache memory 1021 b (cache device name “CACHE-B”) are assigned to the storage node 100 c (storage node name “SN-C”).

The following is understood from FIGS. 22A and 22B. Before the storage node 100 a fails, the assignment is set in the connection state table 560 such that, as shown in FIG. 22A, the logical units 211 a and 212 b and the cache memory 1021 b are assigned to the storage node 100 a. After the storage node 100 a has failed, the assignment of the logical units 211 a and 212 b and the cache memory 1021 b to the storage node 100 c is reflected in the connection state table 570 as shown in FIG. 22B.

By using the storage system described above, in the event of a failure of the storage nodes 100 a to 100 d, data not yet written into the storage devices 200 a to 200 d is passed to another normal storage node and is written into the storage devices 200 a to 200 d because the data is backed up in corresponding one of the cache memories 1021 a to 1021 d. Therefore, data loss can be avoided. Other data having been already written into the storage devices 200 a to 200 d by the failed storage node is passed to the other normal storage node through reassignment of the storage devices 200 a to 200 d and the cache memories 1021 a to 1021 d, which is performed by the control nodes 500 a and 500 b having detected the failure. Accordingly, the operation of the storage system can be maintained continuously.

Further, in the ordinary mode, data is backed up in one of the cache memories 1021 a to 1021 d by writing the data in an asynchronous manner. More specifically, in the ordinary mode, the writing of data from the terminal device 400 a is executed such that the data is temporarily stored in the cache memory 1021 b and the completion of this process is then indicated to the terminal device 400 a without waiting for the completion of the writing into the storage device 200 a. As a result, a quicker response to the terminal devices 400 a to 400 d can be obtained in spite of backing up the data.

The storage nodes 100 a to 100 d in this embodiment not only perform backup of the data having been written into the storage devices 200 a to 200 d and management of the storage device and the cache memory which are newly assigned thereto, but also execute the process of usually writing data into the storage devices 200 a to 200 d by themselves. However, the functional arrangement is not limited to that described above in the embodiment. For example, the process of usually writing data into the storage devices 200 a to 200 d may be executed by another device, and the storage nodes 100 a to 100 d may just perform backup of the written data and management of the storage device and the cache memory which are newly assigned thereto, without executing the process of usually writing data into the storage devices 200 a to 200 d.

Modification of this Embodiment

A modification of this embodiment will be described below with reference to the drawing. The difference between the above-described embodiment and the modification is described primarily regarding the difference from the exemplary configuration shown in FIG. 19. Similar components to those in FIG. 19 are denoted by the same characters. The storage system according to the modification of this embodiment differs in that two types of the storage devices are prepared and used separately. More specifically, the storage system of the modification includes main storage devices for storing data, and backup storage devices serving as cache devices, instead of the cache memories, which back up the data written into the main storage devices. Another difference is that the main storage devices and the backup storage devices are interconnected by using a fiber channel (FC) to constitute a storage area network (SAN). FIG. 23 is a block diagram showing an exemplary configuration of the storage system according to the modification of this embodiment.

The storage system according to the modification of this embodiment, shown in FIG. 23, includes a plurality (e.g., four) of storage nodes 2100 a, 2100 b, 2100 c and 2100 d, at least one control node, e.g., two control nodes 500 a and 500 b, and at least one (e.g., one) terminal device 400 a, which are interconnected via a network 60 through IP switches 2700 a and 2700 b.

Further, a plurality (e.g., four) of main storage devices 2200 a, 2200 b, 2200 c and 2200 d, and a plurality (e.g., four) of backup storage devices 2600 a, 2600 b, 2600 c and 2600 d are interconnected via a fiber channel (FC) through FC switches 2800 a and 2800 b, thereby constituting a SAN 70. The storage node 2100 a is connected to the FC switches 2800 a and 2800 b of the SAN 70. Though not shown, the other storage nodes 2100 b to 2100 d are each also connected to the FC switches 2800 a and 2800 b of the SAN 70 similarly to the storage node 2100 a.

The storage nodes 2100 a to 2100 d, the control nodes 500 a and 500 b, and the terminal device 400 a are constituted by computers which are interconnected via the network 60 based on IP (Internet Protocol).

The storage nodes 2100 a to 2100 d, the control nodes 500 a and 500 b, and the terminal device 400 a have each two network interfaces (not shown). Those two network interfaces are connected to the IP switches 2700 a and 2700 b so that each path is constituted in dual redundancy.

Also, each of the storage nodes 2100 a to 2100 d has two HBAs (host bus adaptors) (not shown). Those HBAs are connected to the FC switches 2800 a and 2800 b so that the connection to the SAN 70 is constituted in dual redundancy.

The HBAs are interface controllers for connecting the SAN 70 to the storage nodes 2100 a to 2100 d. In this embodiment, the SAN 70 made up of the main storage devices 2200 a to 2200 d, the backup storage devices 2600 a to 2600 d, etc. is connected to the storage node 2100 a to 2100 d via the fiber channel.

With the above-described arrangement, the storage nodes 2100 a and 2100 d can read and write data from and into the main storage devices 2200 a to 2200 d and the backup storage devices 2600 a to 2600 d, which constitute the SAN 70. More specifically, the storage nodes 2100 a to 2100 d manage data stored in the logical units of the main storage devices 2200 a to 2200 d which are assigned thereto by the control nodes 500 a and 500 b, and provide the data under the management to the terminal device 400 a via the network 60.

In addition, the storage nodes 2100 a to 2100 d provide, to the terminal device 400 a, information processing service utilizing the data stored in the main storage devices 2200 a to 2200 d which are managed respectively by the storage nodes 2100 a to 2100 d. Stated another way, each of the storage nodes 2100 a to 2100 d executes a predetermined program in response to a request transmitted from the terminal device 400 a via the network 60 and then executes reading/writing of data and transmission/reception of data in response to the request from the terminal device 400 a.

More specifically, upon receiving a data read request from the terminal device 400 a via the network 60, each of the storage nodes 2100 a to 2100 d reads the requested data from one of the main storage devices 2200 a to 2200 d, which is assigned to the relevant storage node, through the HBA of the relevant storage node and the FC switches 2800 a and 2800 b, and then makes a response to the terminal device 400 a having transmitted the read request. Also, upon receiving a data write request from the terminal device 400 a via the network 60, each of the storage nodes 2100 a to 2100 d writes the data into one of the main storage devices 2200 a to 2200 d, which is assigned to the relevant storage node, through the fiber channel and the FC switches 2800 a and 2800 b. When the writing of the data is completed, each of the storage nodes 2100 a to 2100 d makes a response to the terminal device 400 a for indicating the completion of the data writing.

The control nodes 500 a and 500 b are computers for managing the storage nodes 2100 a to 2100 d, the main storage devices 2200 a to 2200 d, the backup storage devices 2600 a to 2600 d. More specifically, the control nodes 500 a and 500 b in this embodiment assign the logical units of the main storage devices 2200 a to 2200 d to the storage nodes 2100 a to 2100 d. Further, the control nodes 500 a and 500 b assign the logical units of the backup storage devices 2600 a to 2600 d to the storage nodes 2100 a to 2100 d in order to temporarily store data for backup when the data is written into the main storage devices 2200 a to 2200 d.

In addition, the control nodes 500 a and 500 b acquire information regarding data management from the storage nodes 2100 a to 2100 d and update the information as required.

More specifically, each of the control nodes 500 a and 500 b holds, in the form of, e.g., a connection state table, the setting of assignment of the main storage devices 2200 a to 2200 d and the backup storage devices 2600 a to 2600 d to the storage nodes 2100 a to 2100 d. When the assignment is performed at the startup, or when the assignment is updated with new assignment upon, e.g., detection of a failure, the control nodes 500 a and 500 b update the connection state table and record the latest assignment setting.

To increase reliability, the control nodes 500 a and 500 b are constituted in dual redundancy and are interconnected to mutually confirm their live states by using a mutual monitoring system such that one control node can detect an abnormality, e.g., a failure, of the other control node. Further, one (e.g., 500 a) of the control nodes 500 a and 500 b is a currently operating system and is operated in the usual state, and the other (e.g., the control node 500 b) is a standby system and is held standby in the usual state. For example, when the control node 500 a serving as the currently operating system has failed, the control node 500 b maintained as the standby system is operated instead of the control node 500 a to succeed the processing executed by the control node 500 a.

When the storage system is started up, the control nodes 500 a and 500 b assign the main storage devices 2200 a to 2200 d and the backup storage devices 2600 a to 2600 d to the storage nodes 2100 a to 2100 d.

Further, the control nodes 500 a and 500 b can detect a failure of the storage nodes 2100 a to 2100 d and a failure of the backup storage devices 2600 a to 2600 d. Upon detecting a failure of the storage nodes 2100 a to 2100 d, the control nodes 500 a and 500 b newly assign the storage device and the high-speed storage device, which have been so far assigned to the failed storage node, to another storage node.

The terminal device 400 a is a computer operated by a user of the storage system. The user of the storage system can access the storage nodes 2100 a to 2100 d and can perform reading/writing of data stored in the storage devices by operating the terminal device 400 a.

The SAN 70 is a network interconnecting the storage devices, such as the main storage devices 2200 a to 2200 d and the backup storage devices 2600 a to 2600 d, via a large-capacity communication path, such as the fiber channel, so that high-speed data access can be performed. With the SAN 70, data can be transferred without exerting a processing load on a computer which tries to access the SAN 70. Accordingly, a large amount of data can be transferred with high efficiency by cooperation of servers with the SAN 70 used in a communication path.

The main storage devices 2200 a to 2200 d are each a disk array device having a plurality of built-in HDDs based on RAID5, and they have respective logical units (LU). The main storage devices 2200 a to 2200 d constitute the SAN 70 and are connected to the storage nodes 2100 a to 2100 d through iSCSI. The storage nodes to which the logical units of the main storage devices 2200 a to 2200 d and the logical units of the backup storage devices 2600 a to 2600 d are assigned can exclusively access the assigned logical units.

The main storage devices 2200 a to 2200 d are each preferably made of a large-capacity HDD, such as a SATA (Serial ATA (Advanced Technology Attachment)). Only the storage node assigned by the control nodes 500 a and 500 b can exclusively access corresponding ones of the main storage devices 2200 a to 2200 d.

In this modification, the main storage devices 2200 a to 2200 d provide disk management service of RAID5. The main storage devices 2200 a to 2200 d are not limited to RAID5 and they may be constituted by using another type of RAID. As an alternative, a disk array of the main storage device may be constituted by using another suitable technique other than RAID. Further, the main storage devices 2200 a to 2200 d may be each constituted by a single hard disk without using a disk array, or another suitable storage device.

Each of the main storage devices 2200 a to 2200 d has two fiber channel interfaces connected to the fiber channel. Those two fiber channel interfaces are connected to the FC switches 2800 a and 2800 b, respectively.

The backup storage devices 2600 a to 2600 d are each a disk array device of RAID5 which is constituted by interconnecting a plurality of disk devices via a fiber channel, and they have respective logical units (LU). A high-speed disk drive, e.g., a semiconductor disk or a SAS (Serial Attached SCSI), is preferably used as each of the backup storage devices 2600 a to 2600 d. The use of such a high-speed disk drive can constitute a high-speed disk array device and can increase the operation speed when data is backed up and recovered by the backup storage devices 2600 a to 2600 d in the storage system.

Each of the backup storage devices 2600 a to 2600 d has two fiber channel interfaces connected to the fiber channel. Those two fiber channel interfaces are connected to the FC switches 2800 a and 2800 b, respectively.

The backup storage devices 2600 a to 2600 d are assigned to the storage nodes 2100 a to 2100 d by the control nodes 500 a and 500 b. Further, the backup storage devices 2600 a to 2600 d store, for backup, data transmitted from the assigned storage nodes to be written into the main storage devices 2200 a to 2200 d.

With the arrangement described above, for example, when the main storage device 2200 a and the backup storage device 2600 a are assigned to the storage node 2100 a, data loss can be avoided even if the storage node 2100 a fails. Stated another way, by completely writing, into the backup storage device 2600 a, data which is already transmitted from the terminal device 400 a to the storage node 2100 a, but which is not yet completely written into the main storage device 2200 a, the data is not lost even with the failure of the storage node 2100 a.

In comparison with the case where the backup storage device for temporarily backing up data at the time of writing the data into the storage device is included in the storage node which executes the process of writing the data into the relevant backup storage device, therefore, an advantage can be obtained in avoiding data loss caused by a failure of the relevant storage node.

The backup storage devices 2600 a to 2600 d used in the modification of this embodiment are not limited to RAID5 and they may be constituted by using another type of RAID. As an alternative, a disk array of the backup storage device may be constituted by using another suitable technique other than RAID. Further, the backup storage devices 2600 a to 2600 d may be each constituted by a single hard disk without using a disk array, or another suitable storage device.

Instead of the backup storage devices 2600 a to 2600 d in the modification of this embodiment, other cache devices such as cache memories are also usable. In such a case, the cache devices may be included in the storage nodes 2100 a to 2100 d as with the cache memories 1021 a to 1021 d in the above-described embodiment.

Also, while in the modification of this embodiment the SAN 70 is constituted by the main storage devices 2200 a to 2200 d and the backup storage devices 2600 a to 2600 d, the arrangement of the storage devices is not limited to the above-described modification. For example, the main storage devices 2200 a to 2200 d and the backup storage devices 2600 a to 2600 d may be connected to the storage nodes 2100 a to 2100 d via a network based on IP, as in the above-described embodiment, without using the SAN. Alternatively, the main storage devices 2200 a to 2200 d and the backup storage devices 2600 a to 2600 d may be constituted as separate SANs, i.e., a SAN made up of the main storage devices 2200 a to 2200 d and a SAN made up of the backup storage devices 2600 a to 2600 d. Further, instead of constituting both groups of the main storage devices 2200 a to 2200 d and the backup storage devices 2600 a to 2600 d as two SANs, only one group may be constituted as a SAN.

The thus-constructed storage system according to the modification can also realize similar operation and advantages to those obtained with the above-described embodiment.

While the storage management device, the storage system control device, the storage management program, and the storage system according to the present invention have been described in connection with the illustrated embodiment and modification, the above description merely explains the principle of the present invention. The present invention is not limited to constructions and application examples exactly as per illustrated and described above. The present invention can be modified and altered in various ways by those skilled in the art. Corresponding modifications and equivalents can be all included in the scope of the present invention, which is defined by attached claims and equivalents thereof, and individual components can be replaced with any other suitable ones having similar functions. Also, any other suitable components or steps may be added to the present invention. Further, the present invention may be implemented in a combination of two or more constituent elements (features) in the above-described embodiment and modification.

The above-described processing functions can be realized with computers. In such a case, programs are provided which describe processing details of the functions to be executed by the storage nodes 100 a to 100 d, the management node 300, the terminal devices 400 a to 400 c, and the control nodes 500 a and 500 b. The above-described processing functions are achieved with the computers by executing the programs in the computers.

The programs describing the processing details can be recorded on a computer readable recording medium. Examples of the computer readable recording medium include a magnetic recording device, an optical disk, a magneto-optical recording medium, and a semiconductor memory. The magnetic recording device may be, e.g., a HDD, a flexible disk (FD), or a magnetic tape (MT). The optical disk may be, e.g., a DVD (Digital Versatile Disk), a DVD-RAM, a CD-ROM (Compact Disc-Read Only Memory), or a CD-R (Recordable)/RW (ReWritable) disk. The magneto-optical recording medium may be, e.g., an MO (Magneto-Optical) disk.

For distributing the programs, a portable recording medium, such as a DVD or a CD-ROM, recording the program is put into a commercial market. Alternatively, the program may be stored in a server computer such that the program is transferred from the server computer to another computer via a network.

A computer executing the program loads, into its memory, the program recorded on the portable recording medium or the program transferred from the server computer. Then, the computer reads the program from its memory and executes processing in accordance with the program. The computer can also directly read the program from the portable recording medium and executes processing in accordance with the program. In addition, when the program is transferred from the server computer, the computer can execute processing in accordance with the program each time it receives the program.

The many features and advantages of the embodiments are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the embodiments that fall within the true spirit and scope thereof. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the inventive embodiments to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope thereof. 

1. A storage management device used in a storage system, which stores data in a plurality of storage devices interconnected via a network in a distributed manner, for managing the storage device, the storage management device comprising: management unit managing one of the storage devices assigned thereto and temporary storage unit assigned thereto, the management unit comprising; backup unit, when the data is written into the storage device, storing the data in the previously assigned temporary storage unit before the writing of the data into the storage device is completed; and take-over unit, when data which is already stored in the temporary storage unit, but which is not yet written into the storage device exists at timing the storage device and the temporary storage unit are assigned, writing the not-yet-written data into the storage device.
 2. The storage management device according to claim 1, wherein the storage device and the temporary storage unit are assigned by a storage system control device for controlling the storage system by managing the storage management device, and wherein when the data which is already stored in the temporary storage unit, but which is not yet written into the storage device exists at timing the storage device and the temporary storage unit are assigned by the storage system control device, the take-over unit writes the not-yet-written data into the storage device.
 3. The storage management device according to claim 2, wherein after the writing of the data into the storage device has been completed, the backup unit erases the data having been written into the temporary storage unit.
 4. The storage management device according to claim 2, wherein when the data which is already stored in the temporary storage unit, but which is not yet written into the storage device exists at timing the storage device and the temporary storage unit are assigned, the take-over unit reads the not-yet-written data from the temporary storage unit and writes the not-yet-written data into the storage device.
 5. The storage management device according to claim 2, further comprising internal storage unit for storing data not yet completely written when the data is written into the storage device, wherein the backup unit stores the not-yet-written data in the internal storage unit before the writing of the data into the storage device is completed, and wherein when data which is already stored in the internal storage unit, but which is not yet written into the storage device exists at timing the temporary storage unit is assigned, the take-over unit reads the data from the internal storage unit and writes the not-yet-written data into the storage device.
 6. The storage management device according to claim 5, wherein after the writing of the data into the storage device has been completed, the backup unit erases the data having been written into the internal storage unit.
 7. The storage management device according to claim 2, wherein, after the data writing into the temporary storage unit has been completed, but before the data writing into the storage unit has been completed, the backup unit outputs a write completion response notifying that the data writing into the temporary storage unit has been completed.
 8. The storage management device according to claim 2, wherein, in accordance with an instruction from the storage system control device, the backup unit is able to, when the data is written into the storage device, selectively change between an ordinary mode in which the data is stored in the assigned temporary storage unit before completion of the data writing into the storage device, and a synchronous mode in which the data is written into the storage device without storing the data in the temporary storage unit.
 9. The storage management device according to claim 2, wherein the storage management device includes the temporary storage unit therein.
 10. The storage management device according to claim 9, wherein the temporary storage unit is assigned to the storage management device other than the storage management device which includes the temporary storage unit therein.
 11. A storage system control device for controlling a storage system which stores data in a plurality of storage devices, the storage system control device controlling the storage system by managing a storage management device which is capable of managing the storage device and capable of, when the data is written into the storage device, storing the data in a previously assigned temporary storage unit before the writing of the data into the storage device is completed, the storage system control device comprising; failure detection unit for detecting a failure of the storage management device; and assignment unit for assigning the storage device and the temporary storage unit to the storage management device, wherein when the failure of the storage management device is detected by the failure detection unit, the assignment unit assigns, to another storage management device, the storage device and the temporary storage unit, which have been assigned to the storage management device causing the failure detected by the failure detection failure.
 12. The storage system control device according to claim 11, wherein the failure detection unit further detects a failure of the temporary storage unit, and wherein when the failure of the temporary storage unit is detected by the failure detection unit, the assignment unit assigns another temporary storage unit to the storage management device to which the temporary storage unit causing the failure detected by the failure detection failure has been assigned.
 13. The storage system control device according to claim 11, wherein the temporary storage unit is included in the storage management device, and wherein when the temporary storage unit included in the storage management device is assigned to one storage management device including one temporary storage unit, the assignment unit assigns, to the one storage management device, the temporary storage unit included in the storage management device other than the one storage management device.
 14. A recording medium storing a storage management program causing a computer to execute, in a storage system which stores data in a plurality of storage devices interconnected via a network in a distributed manner, a process of managing the storage device, the program causing: the computer to function as management unit for managing one of the storage devices assigned thereto and temporary storage unit assigned thereto, and the management unit to function as: backup unit, when the data is written into the storage device, storing the data in the previously assigned temporary storage unit before the writing of the data into the storage device is completed; and take-over unit, when data which is already stored in the temporary storage unit, but which is not yet written into the storage device exists at timing the storage device and the temporary storage means are assigned, writing the not-yet-written data into the storage device.
 15. A storage system for storing data in a plurality of storage devices interconnected via a network in a distributed manner, the storage system comprising: a storage management device including management means for managing one of the storage devices assigned thereto and temporary storage unit assigned thereto; and a storage system control device for controlling the storage system by managing the storage management device, the storage system control device including assignment unit for assigning the storage device and the temporary storage unit to the storage management device, the management means comprising; backup unit for, when the data is written into the storage device, storing the data in the previously assigned temporary storage unit before the writing of the data into the storage device is completed; and take-over unit for, when data which is already stored in the temporary storage unit, but which is not yet written into the storage device exists at timing the storage device and the temporary storage unit are assigned, writing the not-yet-written data into the storage device. 